我正在尝试更新安装了 Ubuntu 16.04 的计算机，但是在尝试使用命令升级时出现身份验证错误do-release-upgrade。
$ sudo do-release-upgrade
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [819 B]
Get:2 Upgrade tool [1,248 kB]
Fetched 1,248 kB in 0s (0 B/s)
authenticate 'bionic.tar.gz' against 'bionic.tar.gz.gpg'
gpg exited 2
Debug information:
gpg: Signature made Tue 05 Jan 2021 07:18:49 PM EST using RSA key ID C0B21F32
gpg: Can't check signature: public key not found
Authentication failed
Authenticating the upgrade failed. There may be a problem with the network or with the server.
我不确定网络问题可能是什么，我可以正常连接到互联网。