Here's a couple things to try:
- Make sure the server you are running OctoPrint on is connected to the internet and can reach
plugins.octoprint.org. Try SSHing in and doing a
ping google.comand a
ping plugins.octoprint.org. Both should succeed without any errors.
- Make sure that OctoPrint's connectivity check is setup and working correctly.
- Finally, make sure your server's local date and time match reality, otherwise the https connection to the plugin repository will fail. SSH into the machine and issue
date. It should report the current date in UTC.