How do I fix macOS Mojave httpd syntax error in a localhost setup?
It sounds like you do actually not have any configuration files in the /private/etc/apache2/users/ directory, and thus you cannot include them - therefore the configuration check fails.
If you have users on your system with shared a shared Sites folder, you would usually have configuration files in this directory named username.conf with the following contents:
<Directory "/Users/username/Sites/">
Options Indexes MultiViews
Require all granted
</Directory>
(Note: You'll need to replace "username" in the filename and the contents with the actual name of the user)