We are trying to work out what Mueller intended to convey when his used the 'not confident he is innocent' formulation in his recent public statement.
Then let me work it out for you. The Democrats were disappointed with the initial report. Nadler probably went to Mueller and said we need you to testify before congress and help us take Trump down. Mueller did not want to testify before congress because he didn't want congressional Republicans investigating the investigation. When Mueller declined, Nadler said, then you have to give us something more to help our cause. Hence we get the statement, "We can't know for sure that Trump did not commit a crime."
Imagine if the DoJ could indict a sitting president. Our elections would be meaningless. When a president is elected who is not supported by the DoJ, any one of them could just make up a case and render the executive branch of government impotent.