“Insufficient evidence” = not guilty
That's right, but Mueller did not say they had insufficient evidence. I have not read it, but apparently his report has about 200 pages of evidence of obstruction of justice.
"Obstruction of justice" doesn't mean much to most people, but prosecutors take it seriously because they don't think it can be seen as acceptable to just block or interfere with criminal investigations.
So Mueller feels that crimes were committed (obstruction of justice). He couldn't indict a sitting president, and he didn't want to verbally indict him. But he did want to point out that it is not true that they had "insufficient evidence".