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It produced this output:
Saving debug log to /var/log/letsencrypt/letsencrypt.log
The nginx plugin is not working; there may be problems with your existing configuration.
The error was: NoInstallationError("Could not find a usable 'nginx' binary. Ensure nginx exists, the binary is executable, and your PATH is set correctly.")
The --nginx plugin makes assumptions about the location of the nginx binary and config path based on the o/s it runs on. I am guessing the casaos path isn't the standard default path.
You have two options. Personally, I'd lean towards using --webroot and a --deploy-hook instead of --nginx.
The --nginx option works by making a temp change to your nginx config, reloading it to effect those changes, making the cert request, undoing the temp config change and reloading nginx again to remove them from actively running nginx. This requires a close interaction of your nginx system and Certbot --nginx plugin.
The --webroot option just writes a challenge token file to the path you give with --webroot-path. The cert request is made and the token file removed after. The --deploy-hook can reload nginx whenever you get a fresh cert (not every time Certbot runs).
To get the --nginx option to work on "non-standard" installs review the Certbot docs (here) for
Another key difference is that --nginx option will create a server block for port 443. With --webroot you need to make one manually after you get a cert. Certbot uses the Mozilla recommendations here: Mozilla SSL Configuration Generator
