Cannot upgrade from Ubuntu 14.04 to 16.04

I have the following Ubuntu version: 14.04.6 LTS and I'm trying to upgrade to 16.04 with

sudo do-release-upgrade

This command fails reporting:

An unresolvable problem occurred while calculating the upgrade:
E:Unable to correct problems, you have held broken packages.
This can be caused by:  
 * Upgrading to a pre-release version of Ubuntu  
 * Running the current pre-release version of Ubuntu  
 * Unofficial software packages not provided by Ubuntu

And /var/log/dist-upgrade/main.log says:

2019-09-06 17:15:26,873 ERROR Dist-upgrade failed: 'E:Error, pkgProblemResolver:::Resolve generated breaks, this may be caused by held packages

My question is: how to identify and delete these broken deps?

Thank you.