Trump can release his tax returns if he wants to. What he is telling you is that he doesn't want to release them until after the audit because they might change during the audit. Lawyers and accountants who handle the audits would never recommend releasing tax returns during an audit, because they might change as a result of the audit. Nixon's changed during the audit.
For Democrats and Trump opponents to be wailing about tax returns is laughable, especially when they rely on "transparency" as their motivation. If these people were interested in "transparency", we wouldn't have 30 million illegal aliens roaming around the country, $20 trillion in national debt for which no one can find a bona fide cause or benefit to explain, and an $800 trillion a year trade deficit tht politicians who disclosed their tax returns claim is "Good for America".