The system isn't smart enough to know to whom that the accept/reject selection pertains, even if you have but one pending friend request. You have to indicate to which of the potentially many friend requests your accept/reject selection pertains.
Apparently you have one pending friend request that you would like to get rid of. Bring up the manage friend requests window. You should see a check box next to the user who has a pending friend request. Click that check box for that and then click the button that rejects/accepts selected pending friend requests.