Here's a hint from the postfix website
In order to install or upgrade Postfix
Create a user "postfix" with a unique user id and group id. Preferably, this is an account that no-one can log into. The account does not need an executable login shell, and needs no existing home directory.
postfix:*:12345:12345: postfix: /no/where:/no/shell
Make sure there is a corresponding alias in /etc/aliases
postfix: root Create a group "postdrop" with a group id that is not used by any other user account. Not even by the postfix user account.
I says postdrop group should not be used by any other user account.
That's also what your error message says.
Good luck with the rest.