Comment 7 for bug 1744722

The problem is that the release upgrader does not find any candidate for any base meta package because the release has been removed from archive.ubuntu.com. Then, when it disables third party repositories (ie non official Ubuntu repositories or whitelisted third party), it tries to be kind and adds invalid entries for main (in DistUpgradeController.py lines 730-748) These entries are invalid because it takes them from the entry that has been disabled.

for example in your case this line has been added:
deb http://archive.ubuntu.com/ubuntu stable main # auto generated by ubuntu-release-upgrader