Why doesn't Apache start when I turn on mpm_worker?

Question

Apache HTTP Server

I want to change mpm_prefork to mpm_worker. Registered through the a2enmod and a2dismod commands. OS Ubuntu 16.04.
After switching and rebooting, Apache throws an error:
Job for apache2.service failed because the control process exited with error code. See "systemctl status apache2.service" and "journalctl -xe" for details. Job for apache2.service failed because the control process exited with error code. See "systemctl status apache2.service" and "journalctl -xe" for details.
Opened systemctl status apache2.service:
* apache2.service - LSB: Apache2 web server
Loaded: loaded (/etc/init.d/apache2; bad; vendor preset: enabled)
Drop-In: /lib/systemd/system/apache2. service.d
`-apache2-systemd.conf
Active: failed (Result: exit-code) since Tue 2018-03-13 13:55:59 EET; 16s ago
Docs: man:systemd-sysv-generator(8)
Process: 22542 ExecStop=/etc/init.d/apache2 stop (code=exited, status=0/SUCCESS)
Process: 22568 ExecStart=/etc/init.d /apache2 start (code=exited, status=1/FAILURE)
I'm a beginner, so I don't know how to switch the mode correctly.