I think your configuration file (/etc/default/useradd) contains reference to group '1000'. you can check this by reading that config file, its text.
so the group '1000' is set as default (probably). but the problem is, you don't have that group. perhaps you should try command 'adduser' as its better than 'useradd' because it has options to define group which the user belongs to. (so it doesn't look for default one)