@ Daafaf: There’s no seperate subscription for Classic, if you’ve bought a WoW subscription it comes with free access to the Classic realms.
Make sure you’re logging into the right game license and region in your battle.net dropdown box. E.g. If subbed to WoW1 BfA, that it also shows WoW 1 (EU) and not WoW 2 (US) or something else.