/etc/ftpusers file exists on your server, cPanel & WHM denies FTP access to all of the cPanel users in that file.
/etc/ftpusers file lists of all of the cPanel users for whom to deny FTP access. To deny FTP access to a user, add the username to the file, with one username per line.
For example, an
/etc/ftpusers file that blocks access to the
user3 users would appear similar to the following example:
user1 user2 user3