Why is it not possible? Maybe in “ordinary times”. Are these “ordinary times”?
“Are these “ordinary times”?”
Well, let us know when you succeed at raiding the whitehut and arresting a sitting president. Don’t be a child plying make believe.
There is a legal way to remove a president. It's called impeachment. Removal requires two thirds of the Senate.
The FBI can't arrest the president. Nor can they issue a gag order against him.
For the FBI (or the military — and they would have to be in the FBI's corner) to arrest the president is the definition of a coup. Just ask General James Mattoon Scott.