Are you referring to Comey?
There is no law that says one can’t indict a president.
There is, it's the Constitution. A DOJ "guideline" on what the Constitution allows is not just a plaque on a wall you get at Cracker Barrel. Here is the
memorandum from 2000 explaining in detail how the Attorneys General arrived at their conclusions. Here's the
short answer:
The indictment or criminal prosecution of a sitting President would unconstitutionally undermine the capacity of the executive branch to perform its constitutionally assigned functions.
It goes back to 1973 (Nixon, iow) and it is
effectively "law" as far as DOJ is concerned and the only way to fight it would be if one had discovered several smoking guns, but even then the argument would be the same; it's up to Congress to try a President.
In order for Mueller to indict Trump, he not only would have triggered a constitutional crisis (which would have bounced to the SCOTUS, who in turn would have immediately invoked everything already outlined in the DOJ guidelines and kicked it back to Congress, wasting a lot of time in the process), he would have had to get approval from AG Barr, which he knew he would never get.
So, yes, he
could have indicted Trump, but it would have been a rogue effort that would simply have been crushed immediately. He knew this, which is precisely why he instead brilliantly booby-trapped his report so that Barr would essentially throw the report into the briar patch.
It was the right thing to do.
What was?
The report itself outlines criminal activity.
Actually, it brilliantly rules out any excuse that the activity is
not criminal (hence the conclusion that he could not exonerate). There is a difference.
Is he above the law or not?
Wrong question. The right question is, which mechanism is the right one to try the President? The Constitution--which is the highest law--says it's Congress through impeachment proceedings.