a2enmod not working on debian (buster)

Also if you used to open your root session with "su", then after upgrading to buster make sure to use "su -" instead, otherwise some commands won't be available (including a2enmod).

It's a PATH variable bug.

Try this:

apt-get install mlocate
updatedb
locate a2enmod

the output is:

/usr/sbin/a2enmod

then

 grep PATH /etc/profile


PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/sbin"
PATH="/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games"
Export PATH

As you can see, /usr/sbin in not in root $PATH

So, edit the first line to

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/sbin"

And then

# . /etc/profile

And then you are OK.