Access to this reaml requires an account with a valid access license. (WOW51900337)

This error indicates that the WoW account you are trying to access doesn't have the required permissions to access this game mode.

Most commonly, this happens when you are trying to access the Public Test Realms (or PTR) and you have multiple PTR accounts. Please, ensure you are selecting the correct PTR account in the menu above the Play button, in the Battle.net desktop application, under Game Version & Account.

Alternatively, after receiving the error and closing the warning, you can select the WoW account in the game client and log in directly.

If no account allows you to access the game version you are trying to access, try creating a new PTR account.

Note: the name or number of the PTR account may not match the name or number of the WoW account you usually play with.