If you cannot add BattleTag or Real ID friends, make sure you are entering your friend’s BattleTag or email address correctly. If you have recently enabled friends settings for your account, it may take up to 3 hours before you can send, receive, and accept new friend invitations. If the problem persists, check the following.
Make sure neither of you has the other one blocked in error, for example, if you ignored each other in-game.
If you blocked each other, remove the block and try adding each other as friends again.
Before you can add friends, recently created Battle.net accounts require you to fill out the first and last name on the Account Details page, as well as a secret question in the Security details.
You can have a maximum of 600 BattleTag or Real ID friends. If you reached 600 friends, you must unfriend someone before you can accept new friend requests.
Friend requests you sent and are still pending will count against your friend list limit.
To cancel pending friend requests:
An account that is restricted by Parental Controls cannot have Real ID friends. If you are under the age of majority, speak to your parents to Update your Parental Control Settings. If your friend is under the age of majority, make sure they do the same.
You can have BattleTag friends even if restricted by Parental Controls.