Hi! I was stuck the same problem as you, looking everywhere for an answer, and yet, nothing. Turns out i found the solution by myself !
First i m gessing you might have a multi sessions licence like me, since the error came directly when you’re supposed to choose on wich pc you want to activate the licence. Well the problem came from here : your licence is already activated on your pc, but still aksing you for being activate.
To redeem the problem, check here : Manage Account >> Edit Profile >> Connexion and Security >> *go all the way down* in Active Sessions > here close every sessions who has the same PC’s name you want to connect (current session), if unsure or still struggling i suggest to click the button “close all sessions“
Now you can retry to open an application and voilà! It’s gone! If not try to refresh your creative cloud window or even reload your account, it might just need to update a bit.