You can't just leave a conditional out there on its own like that; this sort of thing has to start with a rule header, probably "Every turn." (Other rule headers would be things like "Instead of going north in the campground," "Before looking," "Checking taking the tent," like that. The chapter of the documentation on Basic Actions is useful here, and section 9.5 on Every Turn.)
If you fix that, though, you'll find that Inform doesn't understand "Player has not entered the tent for one turn." It looks as though you might be trying to use a formulation like the past and perfect tenses from sections 9.12 and 9.13 of the documentation, but for that you need "We have...." and not "Player has...."
Even so, "Every turn when we have not entered the tent for one turn," though it compiles, doesn't do anything. I'm not sure why, not being awfully confident around this kind of formulation. But you can make it work by relying on the turn count (assuming you want this to happen at the beginning of the game):
Code:
Campground is a room.
The tent is an enterable container in campground.
Every turn when the turn count is greater than 1 and we have not entered the tent:
say "There was a horrible storm that night. Flashes of lightning were everywhere and the rain was falling so hard that you fell into a muddy sink hole and died.";
end the game in death.
Of course this doesn't prevent the player from leaving the tent again. (By the way, this will compile even if you don't indent "end the game in death," since this doesn't have any if-thens inside the rule, but it's probably nice to get into the habit of indenting everything so it's one level in from the thing it's conditional on.)