$ sudo do-release-upgrade -d
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [1,554 B]
Get:2 Upgrade tool [1,342 kB]
Fetched 1,343 kB in 6s (84.2 kB/s)
authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg'
Authentication failed
Authenticating the upgrade failed. There may be a problem with the network or with the server.
仅供参考：我已经试过了sudo apt-get install --reinstall ubuntu-keyring。我还尝试将服务器更改为“主服务器”，但它仍然对我不起作用。