what would cause this cell po447

the cel is on a 2004 subaru wrx/sti

P0447 Evaporative Emission Control System Vent Control Circuit Open

from what i have been reading its saying to check your gas cap and if not the gas cap then what?

the previous owner also says it cannot fuel up at full speed like he has to fill it up really slow. i was talking to someone who was talking about there may be an air vent in the gas tube. has anyone dealt with this before.

also the car does have full exhaust with no cats. except the uppipe is still stock (going to replace) but can that cause any cells as well. there was only 2 codes being 447 and 244 (P0244 Turbocharger Wastegate Solenoid A Range/Performance which i looked up and its saying boost creep?)

any ideas anyone.... i do plan on getting the cobb tuner i have heard that you can program out the cel for having no cats?

i do have a code reader and i reset the codes 224 is gone but 447 is still there.