If you get this error you are probably trying to configure a new ftp user on proftpd.
This error found on proftpd.log refers to UserAlias configuration

UserAlias [ login-user real-user]

add early in your /etc/proftpd/proftpd.conf a line similar to this:

UserAlias ftp-login-name real-unix-user

and that’s it.
Remember to check that the user exists and it’s inside the proper ftp group, based on your proftpd configuration