Apache http/2.0

<IfModule mod_ssl.c>
SSLEngine on
SSLProtocol TLSv1.2
a2enmod http2
vi /etc/apache2/sites-available/domain.xx.vhost
Protocols h2 h2c http/1.1
H2Direct on
DocumentRoot /var/www/domain.xx/web
ServerName domain.xx
service apache2 restart 
tail -f /var/log/apache/error.log 

The mpm module (prefork.c) is not supported by mod_http2. The mpm determines how things are processed in your server. HTTP/2 has more demands in this regard and the currently selected mpm will just not do. This is an advisory warning. Your server will continue to work, but the HTTP/2 protocol will be inactive.

a2enmod proxy_fcgi setenvif
a2enconf php7.2-fpm
a2dismod php7.2
a2dismod mpm_prefork
a2enmod mpm_event
service apache2 restart

See also