Do you have any idea what a prosecutor does? He does not , and should not, make comments or investigate someone's "innocence", or "clear" them of a crime. He simply states whether or not there is sufficient evidence to charge them. You'll note that there are no charges against Trump of collusion or obstruction after 2 years. Hence , not enough evidence to charge him. Mueller making this parting comment along the lines of "If we could've cleared him of obstruction we would have said so" is quite frankly a reckless comment, well outside of his duties, and shows that he is frustrated that after 2 years , millions of dollars, and dozens of partisan attorneys and investigators on staff, he couldnt find enough to bring a charge.
This is not hard to understand stuff unless you have an agenda.