Just lite the title says, for me is impossible to add new players as firends.
Whenever i try to add them, either through the quick “last player option” or the nickname, the game bugs and shuts down.
There is a limit for the number of incomplete (sent but not accepted/rejected) friend requests you can have. It sounds like you have reached that limit.
You can’t see these requests in-game, but you can manage them from the battle.net app.
Upper right corner → Friend Settings → View Sent Friend Requests