The repository 'https://ftp.nluug.nl/db/mariadb/repo/10.4/ubuntu focal Release' no longer has a Release file

[eris]

Try replacing it with:

https://dlm.mariadb.com/repo/mariadb-server/10.x/repo/ubuntu

or

https://dlm.mariadb.com/repo/mariadb-server/10.x/repo/debian

Where 10.x is your version