As the character selection screen shows all characters on the Battle.net account in one convenient location, some may show up as “gray” in the listing and are not available to select to use a Character Service on. To use a service for that character, you will need to select the realm that the character is from first by clicking on the “Realm” selection at the top of the screen and then apply the service to that character.

You can also hover your mouse cursor over a character listed on the right side of the screen to see additional information including the realm it is currently on.