This error can show up if the game data has already been deleted, but the game was not properly uninstalled through the Battle.net app or through the Windows programs list. You may want to check where the PTR was installed to see if the folder still exists, or if there’s still any data in the folder.
If you are trying to uninstall the game through the Windows program list, but the game data doesn’t exist anymore, the best way to fix that would be to first re-install the PTR through the Battle.net app and then uninstall it properly which should remove the entry from the Windows apps list.