CentOS

it prompts "UID xxx is used before, force error for security reason" when I want to add an user with certain uid xxx,(the system needs this uid),and I checked /etc/passwd , no any user uses this uid.

So, how to clear completely used uid ,so I can use that uid.

Command line or GUI?
IIRC UIDs<1000 are reserved for the system, so if
you use GUI, then it may have some built in checks.
Also check /etc/shadow.

(BTW, this isn't the correct forum section.)

I'd start by finding out what uses it.

getent passwd xxx (for some numeric value of xxx)