The gas cap can prevent vandals from siphoning out (stealing) your gas if you have the type with a lock.
You open the gas cap and this will depressurize the gas lines.
gas cap loose..
what kind of car
Unlocking the car doors normally unlocks the cover for the gas cap.
Check your gas cap to see if it is on right. I had that once and it was my gas cap not being on right.
open the gas door on side of car and then twist the (nozzle- cap) and put the gas despencer in it and start pumping
You have to remove the gas cap first.
Vapor locking ...try a new cap
Try changing the gas tank cap, if it does not help you might have to replace the charcoal canister because it does not release gas fume pressure.
It is the cap that screws into the gas tank to keep the gas in, and the elements out.
try pushing gently on the top corner of the gas cap door towards the front of the car
Turn your engine off, check the gas cap and restart your car. If that doesn't work, be more specific about which car you have or check your owners maunal.