no1h
I realize this is an old thread, but I also ran into this issue where apache2 can't connect to the UNIX socket while trying to run srf-ip-conn-srv on a Debian 10 host. I was able to resolve it by changing the location of the api-socket-file setting in the config.json file from /tmp to /var/run, and making the same change in the config.inc.php file in the dashboard folder.
From my research, it seems the issue is that the systemd script for Apache2 that ships with Debian in /lib/systemd/system/apache2.service includes the line PrivateTmp=true. This prevents apache2 from accessing the /tmp folder no matter what permissions you set.
Additionally, as K9AGR pointed out on Ubuntu, to build on Debian you also need the build-essential package in order to compile according to the github instructions.