Did you install mercurial? If not, I suspect that may be the issue (and if it's not THE issue, it is definitely A issue). To do that:

apt update && apt install -y mercurial

And there is actually some Apache config that we'll need. To create that:

cat > /etc/apache2/conf/hg.conf <<EOF
AddHandler cgi-script .cgi
ScriptAliasMatch ^/hg(.*) /usr/lib/cgi-bin/hgweb.cgi/\$1
EOF

And we'll also need to enable that in the main Apache site file:

sed -i "\|Include /etc/apache2/conf/svn.conf|a \    Include /etc/apache2/conf/hg.conf" /etc/apache2/sites-available/revisioncontrol.conf

Then restart Apache:

systemctl restart apache2

Also I did notice in the output that there is also a recommendation to update the documentation. I.e.:

trac-admin "/var/local/lib/trac/hg-helloworld" wiki upgrade

So I suggest that you also run that.

Before you check it again, I suggest clearing out all the Apache logs:

logrotate  /etc/logrotate.d/apache2 --force

Now check it again now. If it still errors, then hopefully more detailed info will be towards the end of the Apache error log (/var/log/apache2/error.log or perhaps one of the other log files in there). Please feel free to post relevant bits here for further assistance.