SHORT ANSWER
I have reviewed over a dozen different translations and I have not found any difference in how the 29th verse is translated. It always is translated to (I will not) so I am lead to believe that your question may be more about the two different responses from the two sons. Let’s take another look at the parable that Jesus tells.
Matthew 21:28-32 (New International Version)
The Parable of the Two Sons
28"What do you think? There was a man who had two sons. He went to the first and said, "Son, go and work today in the vineyard."
29 "I will not," he answered, but later he changed his mind and went.
30"Then the father went to the other son and said the same thing. He answered, "I will, sir," but he did not go.
31"Which of the two did what his father wanted?"
"The first," they answered.
Jesus said to them, "I tell you the truth, the tax collectors and the prostitutes are entering the kingdom of God ahead of you.
32For John came to you to show you the way of righteousness, and you did not believe him, but the tax collectors and the prostitutes did. And even after you saw this, you did not repent and believe him.
Understanding the Parable
Matthew 21:28-32 contains the story of two sons, one who said he would not do the work his father asked of him, yet did, and another who promised to work, but did not. Who are the characters in the Parable of the Two Sons? The father is God. The sons represent two different responses to the work the father asks them to do which corresponds to living God's way of life.
The first son, who flatly refuses to work in the vineyard, represents the weak, foolish, and base of this world. He answers, "I will not," which is a carnal answer from a carnal mind. This is the mind all of us had before God called us out of the world. His answer displays no hypocrisy because he sincerely did not want to come under God's authority. He is guilty of bold rebellion. Ultimately he does respond to the call and does what is right.
The second son, who promises to work yet never shows up, represents hypocrites, those who appear or profess one way but act another. He says, "I go," and makes a promise that he never fulfills—and possibly never intends to fulfill. His word contradicts his performance. While his father is present, he conceals his determination to disobey; he is a liar. As Jesus says in Luke 6:46, "Why do you call me "Lord, Lord," and do not do the things which I say?" This son's guilt combines deception with disobedience. This was a pointed accusation to the chief priests and elders of the people for not doing God’s will.
LONG ANSWER
There is really nothing more to add to the short answer in regards to understanding the verses. Below you will find various translations of verse 29 which all do say I will not. I have also supplied the Original Greek with a word by word lexicon for each word. Please provide the version you have if there is truly an issue with the translation and then we can further discuss it’s method of translation for that specific translation.
If this is more a question of the reliability of the bible, we can open up a thread on Bibliology. Bibliology teaches us that the Bible is inspired, meaning it is "breathed out" by God. A proper Bibliology holds to the inerrancy of Scripture—that the Bible does not contain any errors, contradictions, or discrepancies. There may be problematical translations, but the bible in the original language is without error.
A solid Bibliology helps us to understand how God used the personalities and styles of the human authors of Scripture and still produced His Word and exactly what He wanted to be said. Bibliology enables us to know why other books were excluded from the Bible. For the Christian, the Bible is life itself. Its pages are filled with the very Spirit of God, revealing His heart and mind to us. We must have confidence in it’s origin and content for it to speak to our hearts.
Various Versions of Verse 29
Matthew 21:29 (New International Version)
29"I will not," he answered, but later he changed his mind and went.
Matthew 21:29 (King James Version)
29He answered and said, I will not: but afterward he repented, and went.
New American Standard Bible (©1995)
"And he answered, "I will not"; but afterward he regretted it and went.
King James Bible
He answered and said, I will not: but afterward he repented, and went.
American Standard Version
And he answered and said, I will not: but afterward he repented himself, and went.
Douay-Rheims Bible
And he answering, said: I will not. But afterwards, being moved with repentance, he went.
Darby Bible Translation
And he answering said, I will not; but afterwards repenting himself he went.
English Revised Version
And he answered and said, I will not: but afterward he repented himself, and went.
Webster's Bible Translation
He answered and said, I will not; but afterward he repented, and went.
World English Bible
He answered, "I will not," but afterward he changed his mind, and went.
Young's Literal Translation
And he answering said, "I will not," but at last, having repented, he went.
GREEK Translation and Lexicon
ΚΑΤΑ ΜΑΤΘΑΙΟΝ 21:29 Greek NT: Tischendorf 8th Ed. with Diacritics
ὁ δὲ ἀποκριθεὶς εἶπεν• οὐ θέλω, ὕστερον μεταμεληθεὶς ἀπῆλθεν.
He answered __ and said I will not but afterward he repented and went
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ο definite article - nominative singular masculine
ho ho: the definite article; the (sometimes to be supplied, at others omitted, in English idiom) -- the, this, that, one, he, she, it, etc.
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δε conjunction
de deh: but, and, etc. -- also, and, but, moreover, now (often unexpressed in English).
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αποκριθεις verb - aorist passive deponent participle - nominative singular masculine
apokrinomai ap-ok-ree'-nom-ahee: to conclude for oneself, i.e. (by implication) to respond; by Hebraism to begin to speak (where an address is expected) -- answer.
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ειπεν verb - second aorist active indicative - third person singular
epo ep'-o: to speak or say (by word or writing) -- answer, bid, bring word, call, command, grant, say (on), speak, tell.
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ου particle - nominative
ou oo: no or not -- + long, nay, neither, never, no (man), none, (can-)not, + nothing, + special, un(-worthy), when, + without, + yet but.
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θελω verb - present active indicative - first person singular
thelo thel'-o: by implication, to wish, i.e. be inclined to (sometimes adverbially, gladly); impersonally for the future tense, to be about to; by Hebraism, to delight in
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υστερον adverb
husteron hoos'-ter-on: more lately, i.e. eventually -- afterward, (at the) last (of all).
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δε conjunction
de deh: but, and, etc. -- also, and, but, moreover, now (often unexpressed in English).
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μεταμεληθεις verb - aorist passive deponent participle - nominative singular masculine
metamellomai met-am-el'-lom-ahee: to care afterwards, i.e. regret -- repent (self).
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απηλθεν verb - second aorist active indicative - third person singular
aperchomai ap-erkh'-om-ahee: to go off (i.e. depart), aside (i.e. apart) or behind (i.e. follow), literally or figuratively -- come, depart, go (aside, away, back, out, ... ways), pass away, be past.