Can you clarify, precisely what method you are using to do the 3cx install? sounds like maybe you are using a script-based method on a "bare/base debian install host" maybe? If this is correct, then the error is kind of telling you - "dude you have a service already running and listening on the port I need, please don't do that". ie, maybe review the 3cx install hints, you may need to remove some unnecessary services on your host. If for example your starting point was a pre-cooked VM image by hosting provider - it might have come with apache/some other web daemon pre-installed by default - and specifically you do not want anything present on your 'clean slate' that might interfere with the 3cx install.
in theory you can - carefully - assess what packages you have installed; then remove any unwanted packages - and re-try 3cx installer script -- and it should be able to proceed without error once you get these roadblocks out of the way.
-Tim