14.04 - Failure to upgrade to 16.04

I've been trying to upgrade my computer to Ubuntu 16.04; mine is 14.04.

Unfortunately, I couldn't upgrade it. I always have the error below whether I'm trying to upgrade it using update-manager or using the terminal.

The pictures below show the error.

Image #1

Image #2

It could be you need to switch servers because the one your are downloading from is down

It could be some PPAs were not found. If so try this

Please note the entire error messages aren't visible on your screen snapshot.