www-data is in /etc/group but not /etc/passwd. When I try to create a www-data user with

sudo adduser www-data

I get this message:

adduser: The group `www-data' already exists

This message doesn’t make any sense. When I run

apache2ctl configtest

I get

chown: invalid user: 'www-data'

which also doesn’t make sense.
There is nothing in the apache error.log file.
The apache envars file specifies that the user and group must be www-data:www-data, but Ubuntu will not let me make a user named www-data. I have tried different ways to do this, such as using the system Options dialog, useradd, usermod, etc., with no luck.

Apache2 will not run without www-data:www-data.

Any help, please.