Why can't I put gas in my 2004 Nissan sentra?

I was told to buy a new gas cap so I did but that didn't help the problem

the fuel system is a closed system- but it is vented to prevent a build up of pressure- this is pretty simple, so it sounds like the vent is faulty- it is usually vented through a charcoal canister to prevent gas fumes from reaching the atmosphere- start there-

I assume you take cap off and let air come out before trying to add the gas. Because if not it shouldnt allow gas to enter until then. I notice my car does it more in warmer months than cooler months.

Your canister vent may not be operating properly or the canister is clogged up.

thanks so much!! Now where is this canister vent located??

look at the top of the gas tank- there are hoses coming out- fuel lines- one of them must be a vent hose- follow it- you can just disconnect it to test and see if that relieves the pressure- you must hook it back up, however or your car will smell like gas-