If you follow along with the entire series of article for Virtualmin Installation, you would notice, Virtualmin does not support HTTP2 yet, considering Apache version was still on Apache/2.4.18.
If you have hesitation, run “apache2ctl -V” instead.
[email protected]:~# apache2ctl -V
Server version: Apache/2.4.18 (Ubuntu)
Server built: 2019-04-03T13:34:47
Server's Module Magic Number: 20120211:52
Server loaded: APR 1.6.2, APR-UTIL 1.6.0
Compiled using: APR 1.5.2, APR-UTIL 1.5.4
Architecture: 64-bit
Server MPM: event
threaded: yes (fixed thread count)
forked: yes (variable process count)
Server compiled with....
-D APR_HAS_SENDFILE
-D APR_HAS_MMAP
-D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
-D APR_USE_SYSVSEM_SERIALIZE
-D APR_USE_PTHREAD_SERIALIZE
-D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
-D APR_HAS_OTHER_CHILD
-D AP_HAVE_RELIABLE_PIPED_LOGS
-D DYNAMIC_MODULE_LIMIT=256
-D HTTPD_ROOT="/etc/apache2"
-D SUEXEC_BIN="/usr/lib/apache2/suexec"
-D DEFAULT_PIDLOG="/var/run/apache2.pid"
-D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
-D DEFAULT_ERRORLOG="logs/error_log"
-D AP_TYPES_CONFIG_FILE="mime.types"
-D SERVER_CONFIG_FILE="apache2.conf"
To go about this, let us upgrade our Apache, then enable HTTP2.
What is HTTP/2?
HTTP/2 is the first major HTTP protocol update since 1997 when HTTP/1.1 was first published by the IETF. The new HTTP protocol is needed to keep up with the exponential growth of the web. The successor of HTTP/1.1 brings significant improvement in efficiency, speed and security and is supported by most modern web browsers.
https://tools.keycdn.com/http2-test
Add Ondrej’s Launchpad PPA
sudo add-apt-repository ppa:ondrej/apache2 && sudo apt-get update
Install new version of Apache
sudo apt install apache2
If you are prompted to the question below, just select “N”. This should keep your currently-installed version.
Configuration file '/etc/apache2/apache2.conf'
==> Modified (by you or by a script) since installation.
==> Package distributor has shipped an updated version.
What would you like to do about it ? Your options are:
Y or I : install the package maintainer's version
N or O : keep your currently-installed version
D : show the differences between the versions
Z : start a shell to examine the situation
The default action is to keep your current version.
*** apache2.conf (Y/I/N/O/D/Z) [default=N] ? n
If you wish to see the update version run apache2ctl -V again.
Enable HTTP2 in Virtualmin
Click Webmin > Servers > Apache Webserver > Global Configuration > Configure Apache Modules.

Tick http2 and Click Enable Selected Modules to complete your changes.

Test HTTP2
To test your setup, get your domain that is hosted on your Virtualmin Droplet and visit https://tools.keycdn.com/http2-test. From here, enter your domain and see the result, for example:

On the other hand, you could check your Response Headers on your browser instead. You should have Upgrade: h2,h2c. For example:

At this point, you should be all set.
Source:
How to enable HTTP/2 Protocol on Ubuntu 16.04
Hello. My HTTP/2 test failed at https://www.californiasunspa.com/. Can you help? I can pay.
Hello Danny, Thanks for dropping by.
I understand the urgency, however, better send me more details on what actions you have done so far, considering your inputs are not enough for me to see where to go about your challenges.
To enable http2, I need to do extra:
1. disable both php7.x and mpm_prefork module in Webmin > Servers > Apache Webserver > Global Configuration > Configure Apache Modules.
2. comment out all lines of php_value and php_admin_value in /etc/apache2/sites-enabled/<>.conf
3. restart apache:
sudo apache2ctl configtest
sudo service apache2 restart
As of 2021 July, when I install Virtualmin 6.2.2 on Ubuntu 20.04 LTS, as you describe in “Enable HTTP2 in Virtualmin”, directly tick “http2” in Webmin > Servers > Apache Webserver > Global Configuration > Configure Apache Modules is working, without any other steps.
My issue is that configure apache module is missing entirely in my webmin. Any way to enable that? Centos 8