How to fix `sudo apt-get update` on Ubuntu 16.04?

Question

I'm new to Ubuntu. Got this error while installing ppa:

Answer 1

You seem to have a lot of repositories that are non-functional or missing items for 16.04. You will need to remove or update these items to get rid of these errors.

Go to Settings (the gear icon) and choose Software & Updates. Click on the Other Software tab and remove the entries that are giving you the errors. It will ask for a password. Just enter your login password and once those entries are removed you will no longer get errors.