There are various reasons you may get an error when creating a new user account on WHM. When using cPanel, every user account on an entire server must have 8 unique characters at the start of a username. While it is possible to have a username with less or more than 8 characters, the first 8 must be unique. If you are entering a username where the first 8 characters are already being used by another account on the server you will need to edit the username and try again.
On rare occasions, you may be trying to add an account where the domain name is already on the server. This could be through a different reseller or as an addon domain on another account. Before you can then add this domain as a new user, you will need to get the client to remove the domain from the server.