Error code 0x803F800E is seen with the Xbox game pass. Microsoft has been having problems with the Xbox and eventually they hope to make it more robust.
I unplugged my Xbox and plugged back in. Then signed out of my Xbox gold profile and tried to play an online game knowing it wouldn’t let me. Then I signed back into my account and loaded the game and it worked. Not sure if that’s what fixed it or if it just randomly fixed itself but good luck.
Contact Xbox support and get them to refund your account. Then you should be able to again subscribe to the game pass.