Problem with apt-get update on Ubuntu 20.04 lts

The reason this happens is because not all the mirrors get updated at the same time. For a variety of reasons, updates get rolled out on a schedule and can take a few days for all the mirrors to be in sync.

If you see this problem, the best solution is to just take a nap or wait until the following day. If you'd like confirmation, or to see when your mirrors are updated, you can actually just look here. That will show you the status of mirrors, including how far behind they may be in the sync process.

You can also change your mirrors to new ones, which might be a good idea if it's a major security issue that you're updating to fix.

If you decide to change your mirrors, open 'Software & Updates' from your 'Preferences' and the very first tab has a drop down menu labeled 'Download From:' and pick alternative mirrors. If you're curious, it's generally considered best practice to pick the mirrors that are geologically closest to you.

apt-get clean worked! Thanks for the help from all of you.

In my case, I had to change the Ubuntu Software Update server from Brazil to the USA, then run:

$ apt-get clean
$ apt-get update

And that worked.

On the top right of the screen a red ! was displayed. When I clicked it a message said there is either a network problem or a problem with the sources and I should click Show updates to update the sources manually or check them. When I clicked Show updates the update manager said the software on my pc is actual. First I thought this is just a temporary problem but a few days later there was the same problem.

KGIII's answer helped me. Thanks! He posted, changing the mirrors could solve the problem. For me it works, with the difference that he posted "it's generally considered best practice to pick the mirrors that are geologically closest to you" and I chose Main server.

Then updates have been downloaded!

Today the same error happened again so I set Server for My Country again!
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After that, update download works.