You receive the following error when trying to move an account to another filesystem.
The user is already installed on the only usable partition on this machine
This occurs when there is only one file system recognized as suitable for account home directories.
By default, WHM/cPanel only recognizes file systems mounted to a directory containing "home" as suitable for home directories. You will need to mount the file systems mounted to a directory containing "home" or change what directories WHM/cPanel will consider for home directories. This can be configured in the following location in WHM.
Home » Server Configuration » Basic WebHost Manager® Setup
More information can be found in the documentation linked below.