This error is caused when you have the maximum number of game accounts associated with your Battle.net account. Each Battle.net account is allowed:
- Eight World of Warcraft game accounts.
- One StarCraft II game account.
- One Diablo III game account.
- One Heroes of the Storm game account.
- One Hearthstone game account.
- One Overwatch game account.
Note: If you are trying to purchase a game as a gift, you will need to click the Gift button in the shop.