The trouble deepens! Trump could also be charged with witness tampering. How? p7 of Volume 2, states "At that point , the President engaged in a second phase of conduct, involving public attacks on the investigation, non-public efforts to control it, and efforts in both public and private to encourage witnesses not to cooperate with the investigation." Hello - this is the same sort of conduct that led Republicans to impeach Bill Clinton 20 years ago.
In addition at p132 and p132, Mueller found reason to be concerned that Trump could have potentially influenced a jury trial by making public statements which jurors became aware of.
Just to be clear - very clear - I am not drawing conclusions. The point is, that Mueller had no powers to indict the President. He took the next best course of action which was to gather evidence, to show up the 'smoking gun' and leave it for Congress - with greater powers to investigate.