Are you trying to perform service on the system or do you mean when you press the fuel door button to refuel the car?
The system is designed to remain pressurized in order to extend the life of the gasoline inside the tank. IIRC, when the tank is depressurized, vented vapor/air runs through the EVAP system (into a charcoal canister) in order to capture any gasoline vapor from the vented gases. There was a recall/TSB on the the fuel recovery system, specifically the canister, that I remember having replaced on my 2013; you may want to check on whether this was performed... and if so, properly. I believe everything you are looking for is in the rear of the vehicle; perhaps this link will help: GM Parts Center
When I hit the button to open the fuel door the "waiting to refuel" message stays on forever, 3-4 minutes probably. Then when I remove the gas cap I can hear air escaping. The check engine light is also on, and there are some OBD codes relating to the emission system.
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