@Yan Ubuntu 20.10, along with all flavors like Ubuntu-MATE reached EOL some time ago.
https://ubuntu-mate.community/t/ubuntu-mate-20-10-reaches-end-of-life/24314/17
After a release reaches EOL, it's repositories are moved, meaning the "does not have a Release file" error occurs.
Your issue is you're trying to upgrade a no-longer supported release. You can change the repository to point to it's moved location (there is no automatic tool to do this, it's manual, as you're supposed to release-upgrade to the next release before it's reached EOL thus this issue should not occur). To do this you can refer to EOLUpgrades - Community Help Wiki
Do note: your release-upgrade will also be harder too, as Ubuntu-MATE 20.10 would release-upgrade to 21.04; however that release reached it's EOL only days ago; thus that upgrade is gone.
It maybe easier to upgrade via re-install a supported release. I'd also suggest sticking to LTS or long-term-support release if you don't like release-upgrading every 6-9 months.
Ubuntu-MATE 20.10 meant the 2020-October release; 9 months of life meaning it's EOL was 2021-July..