please tell me what is the problem

Question

Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://ppa.launchpad.net/fossfreedom/packagefixes/ubuntu/dists/xenial/main/source/Sources  404  Not Found
E: Failed to fetch http://ppa.launchpad.net/kroq-gar78/ppa/ubuntu/dists/xenial/main/source/Sources  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.

Answer 1

I'm new to Linux, but I had a similar problem.

Go into

System Settings >> Software & Updates >> click the "Other Software" tab.

Locate:

http://ppa.launchpad.net/fossfreedom/packagefixes/ubuntu/dists/xenial/main/source/Sources

and

http://ppa.launchpad.net/kroq-gar78/ppa/ubuntu/dists/xenial/main/source/Sources

Click the tick box of these two repositories (including any "main" or "source code" you see at the end of these links) so they are unticked.

Close Software & Updates, it'll probably tell you something about "Out of Date." Follow the prompt to updating by clicking the relevant button.

If you get an error, copy it from the tiny text box into a text document and check if it's the same or different. If it's different then your current problem is solved, but other problems remain.

If you get no failed message, then everything is fine.

If it closes down without telling you anything, just be sure and run the update in terminal..

In terminal, type...

sudo apt update

Input your password. When the process is finished,you should get a Y/n prompt. If you do get Y/n, type y and wait for the process to finish.

If none of the same errors at the end of the process, then you're fine.

If everything is fine and updated with none of the same errors, navigate to

/etc/apt/source.list

Open the source file in a text document, select everything inside and copy into a separate text document and save it to your desktop or elsewhere that's easy to navigate to.

This is your backup of source.list. Now, in the source.list file, locate both:

http://ppa.launchpad.net/fossfreedom/packagefixes/ubuntu/dists/xenial/main/source/Sources

http://ppa.launchpad.net/kroq-gar78/ppa/ubuntu/dists/xenial/main/source/Sources

Along with any duplicate links with something at the end of it like "source code," etc. Basically, anything regarding those two links. Save the file.

Test it out, run an update (sudo apt update).

If it says everything is up to date with no error messages of the same, then it's fixed.

The repository links in SOftware & Updates should be gone.

If everything is working fine, delete the backup you made on your desktop of source.list.

If the problem persists, make sure to copy the backup text into the source.list, replacing all text inside

I hope this fixes your problem, it did fix mine =].