Apt-scure(8) Manpage for repository

[tkn]

Are you still running Ubuntu-MATE 18.04 LTS (bionic) ???
That version was dropped from support three years ago.

Ubuntu MATE 18.04 LTS reaches end of life

As of 30 April 2021 Ubuntu MATE 18.04 LTS has reached EOL (End of Life) and is no longer supported. There will be no more updates or security patches for this release. Being a long term release (LTS), official Ubuntu flavors are only supported for 3 years, as opposed to Ubuntu's 5 years. This means MATE components of your system will no longer receive updates after today, but foundational components will continue to receive security updates from Ubuntu. Please note that there is no upgrade pat…

It seems that Open Foam® also dropped support for this version, hence the error.

Best to remove that Open Foam® repository that you added.

Remove Software Repositories from Ubuntu [3 Easy Ways] 😎

Or ,if you still need Open Foam® updates, You might first check which versions of Ubuntu-MATE they are supporting and then:

1. upgrade to the newest Ubuntu-MATE version they support.
2. add their repository after install

Oh, btw, if you decide to upgrade, that old repository will be disabled automatically so in that case you don't have to remove the repository because it will be done for you by the upgrader.