If you can’t add another friend, you’ve most likely reached the maximum number of friends allowed on your account.
What you can do
Check your Friends list and see if it’s already full.
Remove one friend (for example, an inactive player) and try adding the new friend again.
Fully close the game and reopen it to refresh the Friends list.
Important notes
The friends limit is a hard cap. Once you hit it, new invites won’t go through until you free up a slot.
If you removed a friend but still can’t add a new one, the list may need a short time (or a restart) to update.