Hey all,
Going to lock this thread since there are no other causes for this error. This error happens when the Blizzard account used to login with the Battle.net application does not own the game.
Reign of Chaos keys can be claimed here, or if they’re already claimed you can double check the “Games & Subscriptions” page. If you have not claimed your keys and receive an “Already Claimed” message please contact support to see if they are able to recover the original account. Be sure to include your keys in the support site ticket request so the agent can locate the account.
Reforged purchases can be verified in the “Transaction History” page.
I do hope this info helps.