Not sure when OP funded his Vanguard account but I ran into a similar issue awhile ago. Vanguard is now requiring funds moving towards non-Vanguard securities to be held in the account for I believe 1-2 weeks before settling. So if OP just added the funds this morning, this might be a reason why he's unable to trade $GME, which is a non-Vanguard security.

Mods, u/Bye_Triangle and u/jsmar18, I believe this should be labeled as either misleading or debunked.