This error may happen when something interferes with the uninstall process.
How to make a copy of one of your characters in the Public Test Realm and the Beta client
What to do if you want to swap the names between two of your characters.
We can help as long as you have the follower.
You might be in a raid group! We also have some other steps to fix this issue.
What to do if you can't complete one of these quests.