While I'm only adding to the choir of people telling you "of course," since I'm directly the person you're responding to it still feels worth saying: yes, of course, if America and Venezuela went to war, it's completely legal for Venezuela to attempt to kill the U.S. President.

As an American, I certainly hope they would fail. But do I think it's legal? Yes: it's a targeted strike on the leader of an enemy country they'd theoretically be at war with. Do I think it's wise? Well — no, Venezuela has a much smaller military, and assassinating the U.S. President would trigger a massive war that would devastate Venezuela for decades while modestly inconveniencing American taxpayers. But legal? Yes.