https://www.foxnews.com/politics/mueller-issues-clarification-takes-back-bombshell-statement-about-indicting-trump
“I want to add one correction to my testimony this morning," Mueller said. "I want to go back to one thing that was said this morning by Mr. Lieu, who said and I quote, ‘You didn’t charge the President because of the OLC opinion. That is not the correct way to say it. As we say in the report and as I said at the opening, we did not reach a determination as to whether the President committed a crime.”
So the OLC opinion didn't stop them from reaching a conclusion, so it was lack of evidence