Error [sudo /usr/share/jitsi-meet/scripts/install-letsencrypt-cert.sh] TLS certificate problem

I have finished installing Jitsi server in Ubuntu 18.04.05 LTS environment.
However, a problem occurred during the TLS authentication process.
After registering an e-mail to notify the expiration date, the following error occurred.

“Some index files failed to download. They have been ignored, or old ones used instead.”

Can you tell the cause?

no, but this has nothing to do with Jitsi-meet installer; the installer (the certbot script in this case) is using system utility to install some software built and stored by Ubuntu server and the security check has failed. Problem is either with this (Ubuntu) server or with your computer/network access. Try to repeat the offending command (most probably sudo apt update) to see if it fails again, and if yes ask on Ubuntu forums if the korean (I think ?) Ubuntu distribution has a problem.

I tried running sudo apt-get update.
As a result, it was confirmed that the command was not executed as the following result.
E: Could not get lock /var/lib/apt/lists/lock-open (11: Resource temporarily unavailable)
E: Unable to lock directory /var/lib/apt/lists/
Accordingly, it was resolved by using sudo rm /var/lib/apt/lists and succeeded by executing the cerbot script.
Thank you.