Sometimes when creating a cPanel account, you will receive an error like the following.
Unable to create account. (XID vpycqw)test is a reserved username on this system. at /usr/local/cpanel/Whostmgr/Accounts/Create.pm line 623.
This occurs due to there being cPanel usernames that are reserved by cPanel for internal purposes.
To get around this, you will simply need to change your cPanel username to one that is not reserved. Please see the following documentation for more information, and for a list of reserved usernames for cPanel.