When transferring domains using the transfer tool, you may run into an error like this.
Account “cpaneluser”: The account “cpaneluser” already exists on “hostname.server.tld”.
You will first want to make sure this cPanel account doesn't already exist on your server. Do you see it in WHM on the destination server when you navigate to the following location?
"WHM >> Account Information >> List Accounts"
Also, can you navigate to the cPanel user's home directory via the command line?
If so, it is likely this account already exists on the server you are transferring the domain to. If only the username already exists, but it is associated with a different domain name, you can select the "Overwrite" or "Overwrite with delete" option when copying the domain in the transfer tool. This process is described here.
cPanel Docs: Transfer Tool » Accounts
If the same cPanel user is associated with the same domain name on the destination server as it is on the source, you would need to terminate the cPanel account on the destination server first before you can transfer it from the source.
How to terminate an account on your server
If you are getting this error, but are not finding a cPanel account listed with this username, it is likely that this domain existed on your destination server at one time, but was not removed properly. This results in references to this cPanel user existing within various files on the server. You will want to work with a systems administrator if needed to get these references cleaned up from your host. In this case, the following article should help.