Mueller addresses this on page 220 and lays out that while the DOJ cannot get a sitting president on obstruction, congress has the "authority to prohibit a president's corrupt use of his authority"

I know Barr said that he talked to Mueller and Mueller totally said many many a-times that he totally did not consider the opinion that a sitting president cannot be indicted... but he kinda lied there a little bit, it's actually all over the report.