How to Troubleshoot ‘Error Adding Friend’ Problem in Steam
The Steam online portal allows you to buy games, play with friends, share screenshots and custom game assets, and share games. These features are available when you register and purchase a game on this service. To participate in a co-op or cooperative game, you need to add your friend to Steam.
Adding a friend is not difficult, but there can be some problems. Sometimes Steam cannot add friends and displays an error message: “Add Friend Error. The connection between you and this user has been blocked” or “Add Friend Error. Please try again.”