Get:10 https://mirror.download.it/mariadb/repo/10.6/ubuntu focal InRelease
Err:10 https://mirror.download.it/mariadb/repo/10.6/ubuntu focal InRelease
  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
Reading package lists... Done
E: Failed to fetch http://mirror.jaleco.com/mariadb/repo/10.6/ubuntu/dists/focal/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
E: The repository 'http://mirror.jaleco.com/mariadb/repo/10.6/ubuntu focal InRelease' is no longer signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

簡單說明為找不到鏡像網站,該網站已經無法使用了,只好更換網站

進入 /etc/apt/sources.list.d 找到錯誤的list檔案(我的狀況是 mariadb_repo.list 錯誤)

更新為 

deb [arch=amd64] https://ftp.nluug.nl/db/mariadb/repo/10.8/ubuntu/ focal main

重新執行 apt full-upgrade 成功