when I try to "useradd" site owner it says already exists

Hi,

I've got a problem, I am attempting to setup SASL authd with "shadow", so let's say I have a test.domain.com , with owner "test"

but when I "cat /etc/shadow | grep test" , I don't see this user , I can SSH into server if I set SSH allowed , but if I "useradd" or "adduser" , it says user already exists.

OS is centos7.4 with plesk 17.5.3

any help would be much appreciated.