What can I do if a repository/PPA does not have a Release file?

The PPA you've added does not support your version of Ubuntu. 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

Yes, unchecking some tick boxes for Trusty Media and mc3man under

System Settings > Software and Updates > Other Software

did the trick. ppa-purge didn't work for me.

From my personal experience I can recommend to directly contact the PPA owner (if we talk about PPA) to kindly ask him/her to prepare packages for needed (usually newer, so supported release) with steps below:

1. Temporarily disable the PPA in your system (see other answer).
2. Visit PPA page on https://launchpad.net and login to it.
3. Determine the uploader's name in Uploaded by column, click on the name's hyperlink

4. Then click on Contact this user

   

5. And fill the form with all necessary information:

   From: your e-mail will be shown in plain here
   Subject: Packaging of AppName for Ubuntu NN.MM release
   Message:

   Dear PPA owner!

   Thank you for your work on the AppName.
   It would be great if you create the package for it for Ubuntu NN.MM release.

   With best regards,
   user.

6. Wait for an e-mail from PPA owner.

7. Add PPA again to the system when package get published.
8. Enjoy!