Typically this is caused from having many Strangers listed in your "Strangers" tab of your "Friends List", which also takes up Friend slots. If you go into your "Strangers" tab, you can tap on the chat bubble in the player icons which will then present you with the option to delete that Stranger from the list. Doing this will free up a space for you to add another Friend, and you can do this to as many Strangers on your list as you would like in order to free up more Friend slots.
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