Actually, he absolutely tried to interfere. 

Courtesy of NYT:

White House officials asked at least twice in the past month for the key witness against President Trump in the Mueller report, Donald F. McGahn II, to say publicly that he never believed the president obstructed justice, according to two people briefed on the requests.

Mr. McGahn, who was the president’s first White House counsel, declined, one of the people said. His reluctance angered Mr. Trump, who believed that Mr. McGahn showed disloyalty by telling investigators for the special counsel, Robert S. Mueller III, about Mr. Trump’s attempts to maintain control over the Russia investigation.

The White House made one of the requests to Mr. McGahn’s lawyer, William A. Burck, before the Mueller report was released publicly but after the Justice Department gave a copy to Mr. Trump’s lawyers to read. Reading the report, the president’s lawyers saw that Mr. Mueller had left out that Mr. McGahn had told investigators that he believed Mr. Trump never obstructed justice.

White House officials believed that Mr. McGahn’s asserting his belief publicly would calm the president and help the administration push back on the episodes that Mr. Mueller detailed in the obstruction section of the report, said one of the people. Neither would be named describing private conversations involving the White House and spoke on condition of anonymity.

I think my favorite part of this is the fact that Trump told his White House counsel to fully cooperate with the Mueller investigation, assuming that he would, of course, omit or lie about the parts that incriminated him, and then when he realized McGahn actually told the damn truth tried to convince him that he should go out and lie about the truth that he told, only to have him flatly refuse.

Well, I think this makes it clear why Trump is so desperate to keep McGahn from testifying before Congress. 

I think we can only begin to imagine what other tidbits of incriminating information McGahn might be ready to divulge.