He's under audit, I'm sure his attorneys have advised him not to release his returns.
This “excuse” is particularly humorous.
So here is the mindset.
He files his returns with the IRS.
The IRS (which has his information) audits him (or not—all we have are the words of Drumpf—a confirmed liar)
And now he is told to not release his returns (supposedly).
The IRS already has the data. The IRS can not be informed of anything new by releasing them.
So there is no reason to not release them unless he just wants to hide something from the voters.
Which, obviously, is the case.