Over the last 12 months or so, I’ve had to ask some people who what to friend me to give me their battletag to add them, as if I’ve declined their friend request in the past, the game will tell them that their’s still a pending friend request, even though there’s not.
In rare cases, if both players have declined each other in the past, neither is able to add the other, due to phantom friend requests.
The only way to resolve this is through a third party adding one of them, inviting the other to a group and then one of the affected parties clearing the phantom pending requests by blocking and then unblocking, after which, friend requests can be sent as normal.