Hello!
The packages.sury.org has removed the debian 9 packages.

You should use different OS. Debian 9 is End Of Life and PHP packages required for i-MSCP to install on Debian 9 are now impossible to be downloaded.

This should help: [ERROR] iMSCP::Stepper::_callback: The following PHP modules are not installed or not enabled: apc, apcu

You should be able to adapt the commands for PHP5.6

for debian 9 the php packages from sury.org have been removed as per end of june - as stated above.

Maybe (for special situations) the packages from tdrnetworks (made for devuan) may be of help - they are still online (for devuan ascii which is the systemd-less variant of debian 9...

https://pkgs.tdrnetworks.com/apt/devuan ascii main

But there is no apcu_bc package... but the panel works without this module...

But I didn't test, if this works as a replacement...

thanks a million , exactly what i was looking for! actually im upgrading my installation from debian 9 (bit too late, i know ^^) to 10 and hopefully to 11 Will also try getting your fork working on my setup