Hello Kidd.

The fuel cap is the first step in the situation.
The computer has run a test on the evaporative emission system and has failed.
The system will need to be repaired and the code cleared in order to turn it off.

The evaporative system is designed to retain any fuel vapors and burn them when the engine is running. The computer tests this system periodically by pressurizing the system. If the fuel cap is leaking it will fail the test. But, there is a leak elsewhere and will need to be tracked down and repaired.
Evaporative lines, solenoids, fuel tank, carbon canister all are parts of the system that will need to be checked.
Most shops have a "smoke machine" that will introduce smoke and a small amount of pressure to track these leaks down as they can be difficult to find.
You can do a visual inspection to look for anything obvious and also do a sniff check to check for fuel leaks.
Most often in my shop we find a stuck solenoid that is not sealing the system when commanded on by the computer. But this is tricky to test without the use of a scan tool.

If you require further advise on this problem, please ask me here in a comment. Be glad to help.

Thank you for using FixYa.

KL

More than likely the internals of the cap or the seal has failed. Replacement is the fix.