Pages 1 & 2 of Volume II of the Mueller report tells it all.  In those pages, Mueller talks about the DOJ’s policy that a sitting president cannot be indicted.  Mueller is very careful to explain that he’s not recommending indictment because Trump’s not guilty (he most certainly is); he’s not recommending indictment because DOJ CAN’T indict, and due to that fact, a public assertion of guilt cannot be defended in court because there cannot be an indictment.  He even goes so far to say that even a sealed indictment is not practical due to the danger of leaking of that indictment.

So, it’s clear that Trump is guilty of obstruction; it’s also clear that to protect his civil rights, Mueller has declined to recommend indictment and has tossed the responsibility to the Congress (which won’t do anything, of course).