Bistol. Repository no longer has a release file

My OS System is Ubuntu 64-Bit 20.04 LTS gnome version 3.36.2

My problem is Unable to download repository updates but general updates still download its seems I have corrupted the synaptic package manager on my system. My network is working o-k. This appears to have happened during the installation of my Epsom printer XP 455. which is still not working properly. Being new with ubuntu I have no idea how I caused this corruption or how to repair the situation, can you help much appreciated if you can.

When I open the Package Manager And try to download the repository updates it notify the following.

The repository may no longer be available or could not be contacted because of network problems. If available, an older version of the failed index will be used. Otherwise, the repository will be ignored. Check your network connection and ensure the repository address is correct in ‘Repositories’ under ‘Settings’.

The repository ‘http://gb.archive.ubuntu.com/ubuntu disco Release’ does not have a Release file.Updating from such a repository can’t be done securely, and is therefore disabled by default.See apt-secure(8) manpage for repository creation and user configuration details.The repository ‘http://gb.archive.ubuntu.com/ubuntu disco-updates Release’ does not have a Release file.Updating from such a repository can’t be done securely, and is therefore disabled by default.See apt-secure(8) manpage for repository creation and user configuration details.The repository ‘http://security.ubuntu.com/ubuntu disco-security Release’ does not have a Release file.

Thank you to all for any input you may have

Could you post screenshots and copy the exact error messages here instead of paraphrasing them? This would really help us a lot to help you.

Have you tried to switch the repositories in the package manager?
Sometimes this helps out.
Switch to another repo of your country and see, if this works.
Here is a (slightly older) article how to do this.
Or here, from ITSFOSS…

I have no recent Ubuntu on hand, so I can’t deliver screenshots at the moment.

Copy and paste from package manager

The repository ‘http://gb.archive.ubuntu.com/ubuntu disco Release’ does not have a Release file.Updating from such a repository can’t be done securely, and is therefore disabled by default.See apt-secure(8) manpage for repository creation and user configuration details.The repository ‘http://gb.archive.ubuntu.com/ubuntu disco-updates Release’ does not have a Release file.Updating from such a repository can’t be done securely, and is therefore disabled by default.See apt-secure(8) manpage for repository creation and user configuration details.The repository ‘http://security.ubuntu.com/ubuntu disco-security Release’ does not have a Release file.

also: Repository no longer has a release file