According to Bob Woodward’s book, Fear: Trump in the White House, Trump’s lawyers tried to convince Mueller that he couldn’t testify because he is “a pathological liar.” An excerpt reads,

Dowd then explained to Mueller and Quarles why he was trying to keep the president from testifying: “I’m not going to sit there and let him look like an idiot. And you publish that transcript, because everything leaks in Washington, and the guys overseas are going to say, ‘I told you he was an idiot. I told you he was a goddamn dumbbell. What are we dealing with this idiot for?'”

Dowd also dissuaded Trump from testifying:

Don’t testify. It’s either that or an orange jumpsuit…You are not a good witness. Mr. President, I’m afraid I just can’t help you.

The Washington Post – Bob Woodward’s new book reveals a ‘nervous breakdown’ of Trump’s presidency
The Hill – Lawyer told Trump he’s going to jail if he testifies to Mueller: Woodward book
Vanity Fair – EX-TRUMP LAWYER TOLD MUELLER THAT TRUMP IS TOO DUMB TO TESTIFY
The Atlantic – Here Are the Most Extraordinary Quotes From Bob Woodward’s Book