[ubuntu] "Repository does not have a release file" error

If a sudo apt-get update did not do it for you, it might be that some packages have failed to updated to repository-related errors.

For me all of those happened to reside in (Software Updates --> Other Software). You could remove them with "Remove", the cache will be refreshed successfully. Otherwise

sudo apt-get clean
apt-get autoremove 

is something to try.