I've been trying to upgrade my version of Ubuntu 18.04 to 20.04. I've run all the steps from updating and upgrading both apt and apt-get, changing the settings in update manager and all of the steps described in this link. Yet I keep getting the error:

Please install all available updates for your release before upgrading.

Is there something special about trying to upgrade from 18.04 to 20.04 that's causing this error? Do I maybe need to update to 19.04 first?

EDIT: I just noticed something that might be important. When I run sudo apt-get update I get the following notice at the end of the output:

The following packages have been kept back:

  colord gimagereader python-sane sane simple-scan

0 upgraded, 0 newly installed, 0 to remove and 5 not upgraded.

Might that be causing the issue?