Cant add user with Lite User Manager

[SaeedB]

I was trying to add a new user to the system using the Linux Lite User Manager. But after entering my Administrator password, I got this error:



I've tried several times with different usernames and passwords, but I just keep getting this same error; I can't add any user.
Any help?
(I'm on Linux Lite 4.8 btw and everything is up to date).