Indicative of a vent blockage, whether that's the canister vent valve back by the tank, or the canister itself, or evap lines are plugged with charcoal pellets from the canister. You might have an evap code stored. First thing I'd do is see if the canister vent valve is working properly. With no voltage applied (as in key off or connector pulled) you should be able to blow air through it. Applying 12v to it should close it. Repeat in succession to eliminate an intermittent valve. It may have a remote air filter so be sure to check that, too, for blockage.
If that's not the problem then it will be time to dig deeper into the evap system. There are many threads on here about solutions.
Eric shows what the vent valve looks like at about 1:25 or so, and also shows some diag procedures.