Problem Description
it is impossible to install applications
Actual results
(I get an error with the following message:)
Error during installationE: The repository ‘Index of /debian-security testing/updates Release’ no longer has a Release file.
Information
Looks like it’s now called testing-security
http://security-cdn.debian.org/debian-security/dists/
Replace testing with testing-security in all the security lines in /etc/apt/sources.list
You can also remove the lines. See FAQ
Thank you, Joseph! I will try.
Thank you again, Joseph! I corrected the lines in the sources.list. Now programs are installed! And what needs to be corrected that my Freedombox updated? (Post here: Freedombox is not updated)
The same correction might work for system updates too.
After making the above changes, did you try doing a manual update from the Update app under System?
No, I didn 't. But I can try
Three hours update via ssh - and version 20.1! 
Applications are not installed again after the system upgrade. For example, when I try to install Storage, I get a message:
Error during installationE: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable) E: Unable to lock directory /var/lib/apt/lists/
Written for Ubuntu users, but likely helpful here:
[Solved] Could not get lock /var/lib/dpkg/lock Error in Ubuntu
From the link:
You see this error because some other program is trying to update […]. When a command or application is updating the system or installing a new software, it locks the [Debian package manager] file […].
Write down what you try and if it doesn’t work, post back here.
@dgj Thank you for help! Yes, I receive a message on the Storage page that another installation or upgrade is in progress. But I 've been seeing this message for two days. If I reload the Install Storage page, it disappears. I start the installation and get a message again: Error during installationE: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable) E: Unable to lock directory /var/lib/apt/lists/
I get similar messages when I try to install other programs.
Thanks for the link, I 'll study all the methods.