I run the commands

sudo apt-get update, and after, sudo apt-get full-upgrade

or I also run the commands

sudo pkcon refresh, and after, sudo pkcon update

In both cases I can not update the system by throwing the terminal the following error:

Fatal error: E: https://packages.riot.im/debian default/main amd64 element-desktop amd64 v1.7.35 is not (yet) available (404 Not Found [IP: 104.26.5.138 443])

I think is a problem in the expiration of the GPG key of the Debian repository.
How can I solve that?

Talk to your distribution about this. They should provide a new key and instructions on how to update this.

protau wrote:I
Fatal error: E: https://packages.riot.im/debian default/main amd64 element-desktop amd64 v1.7.35 is not (yet) available (404 Not Found [IP: 104.26.5.138 443])

I think is a problem in the expiration of the GPG key of the Debian repository.
How can I solve that?

This error is from some third party repo, and is pretty much saying what the problem is: there is no package of element-desktop with the version 1.7.35 in that archive.
https://packages.riot.im/debian/pool/ma ... t-desktop/

Don't know why it is trying to install this version, but in any case it seems to be a problem on that third party's servers.

Possible workaround is to disable the Element repo, and update, then re-enable it and see if the problem persists.