Dewiz, I found the original quote from London Mayor Sadiq Khan that Trump was responding to:
"My message to Londoners and visitors to our great city is to be calm and vigilant today. You will see an increased police presence today, including armed officers and uniformed officers.
There is no reason to be alarmed by this. We are the safest global city in the world. You saw last night as a consequence of our planning, our preparation, the rehearsals that take place, the swift response from the emergency services tackling the terrorists and also helping the injured.
Fox News did not report that full quotation rather reporting that the London mayor only said "no reason to be alarmed".
Trump then tweeted (maybe after seeing the misleading Fox report, though I, of course, do not know that):
https://twitter.com/realdonaldtrump/status/871328428963901440
So, you can see, Trump mistakenly interpreted Khan's comment to mean there is no need to be concerned about terrorist attacks, when Khan was referring to the visible increase in police activity on the streets of London. Trump got it wrong and may or may not admit that, but you and I know his error.
Trump, of course, has since sent out another tweet doubling down on his position. However, you and I can see reason and the reality of the situation and of his error in logic. We don't have to support his false narrative, am I right!?