I am beginner in debian system. I have run into problem - on my VirtualBox Debian system I have installed mysql and apache2 (two simple commands, what could go wrong), but after installing apache, the default web page on 192.168.0.100 leads to not found status. I have no permissions to access error.log file. Could somebody advise what is the problem? Thanks in advance for your help.
Are you trying to access it from inside the same machine or from a different machine?
If you're getting that error when you try to access the page from the same system, you should make sure Apache is running:
If you're getting that error when you try to access the page from a different machine, either another VM or the host system or another system entirely, you should make sure it isn't being blocked by the firewall:
If you can access the web page from a different machine with the firewall stopped, you'll need to add a rule to allow http traffic through, usually on port 80.
UPDATED AFTER CHAT:
Issue ended up being the wrong IP was being used to reach the VM and the Apache2 instance. User determined this himself during investigation.