Package dependencies cannot be resolved

[mekromancer]

Hi all, please bare with me because I am new to Ubuntu (on 16.04 LTS) but I'm not sure where else to turn to for help. As I try to go through the Software Updater like I routinely do without problem, an error message now pops up every time now and new updates no longer show up on a blank list.

The error message pop-up says:
Package dependencies cannot be resolved
This error could be caused by required additional software packages which are missing or not installable. Furthermore there could be a conflict between software packages which are not allowed to be installed at the same time.

and on the top right corner of the menu bar is a red warning icon with the following message when clicking on it:

'Error: Marking the upgrade (E:Error,pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.)''
This usually means that your installed packages have unmet dependencies

What can I do to resolve this error and continue receiving new updates?

[ian-weisser]

The most common common cause of this particular error message is that you unwisely installed software from a PPA or other non-Ubuntu source.
If that's what you did, then uninstall the software and delete the source.
If you are really sure you didn't do that, then search your memory carefully and tell us exactly what you installed before you encountered the problem.

[Impavidus]

In addition to the above, could you run


sudo apt update
sudo apt upgrade

in a terminal (you won't get any visual feedback when you type the password) and paste the output here? Use code tags, like I did just above. That should give us some error messages telling what's going wrong.