Error “useradd: group '100' does not exist”

I get the above mentioned error when I try to add a user.

By trying

sudo useradd -u 1200 -g test -c 'studente' student

or

sudo useradd anyuser

I get

useradd: group '100' does not exist
useradd: the GROUP= configuration in /etc/default/useradd will be ignored

Content of /etc/default/useradd is

# useradd defaults file
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
CREATE_MAIL_SPOOL=yes

I must have messed up somewhere by creating, modifying and deleting users and groups but I don’t know where.