Fix “The Package Information was Last Updated Days Ago” Error in Ubuntu

If you have noticed, sometimes after you have updated your system, the Ubuntu Update Manager will still show a message saying that “The Package information was last updated X days ago. Press the ‘check’ button below to check for new software updates.“. This was interesting as clicking on the Check button did nothing but the same message was displayed again. After researching a bit, I found out that there were “sources” other than Canonical (Ubuntu’s official sources) which I had added over the time from private PPAs were old and was meant for earlier version of Ubuntu or they were simply broken.