Release File is missing [duplicate]

The PPA you've added does not support your version of Ubuntu, meaning there aren't any packages for your particular release.

For example, the ppa:mc3man/trusty-media PPA is only for Trusty (Ubuntu 14.04) only (trusty-media). Obviously, it has no files for Xenial (16.04). You can check the PPA's Launchpad page to see which versions of Ubuntu are supported.

I'd suggest the following:

• Remove the PPA for an older release. For this example: sudo ppa-purge ppa:mc3man/trusty-media

• If there's a PPA for your curent release, add it. In this case, there's a PPA for Xenial: mc3man/xerus-media. You can add it using sudo add-apt-repository ppa:mc3man/xerus-media

To summarize, you need to remove the added PPA, and use another one that has packages for your particular release.

Remove the PPA with the line:

sudo add-apt-repository --remove ppa:mc3man/trusty-media

Now run

sudo apt-get update