Why do you jump to the conclusion that Trump knows there is a bounty that has already killed soldiers and instructs his charges to do nothing?
Because that is certainly what looks like, given the reporting.
I don't know definitively what Trump knew. I do highly suspect that he is lying about what he knew, given the multiple sources saying he has been briefed, and no credible sources coming forward to say that he hasn't. I think at the very least this was in his intelligent briefings, which he probably didn't read.
But more to the point, we know he knows
now and his response has been to call it a hoax. "There haven't been that many attacks" he said in his tweet, when there were 20 US Soldiers killed in Afghanistan last year.
So now that he knows, what is he doing? The UK has seen enough to say that they will respond.