Check that the apache2 daemon is running by using the following command:
As root user
systemctl status apache2.service
The following result is expected:
● apache2.service - LSB: Apache2 web server
...
Active: active (running) since Mon 2018-11-05 10:10:22 CET; 6h ago
...
The PID number is randomly assigned to an operation and does not have to match the documentation.
If apache2 is not running, use the following command:
As root user
systemctl start apache2.service