Do you have Apache/2.4.17+ ?

• As far as I know, Debian hasn't put Apache 2.4.18 in their stable repo yet, therefore you might want to upgrade your Debian server to a testing version (I strongly recommend doing so, as you will get all your software up to date, which is extremely helpful if you are dealing with some tricky problems that are fixed in the latest version)

• Confirm your Apache version with apache2 -v

Do you have https enabled?

• http 2 works on both http and https, yet most browsers support only http 2 over https

Enable http2 module in Apache

• Try this command a2enmod http2

• If you had other mods to be enabled, replace http2 with the desired module, you can find available modules in /etc/apache2/mods-available/

Add protocols in Apache config file

• vim /etc/apache2/apache2.conf

• vim /etc/apache2/sites-available/default-ssl.conf

Here I have my HTTP2 site