I’m not familiar with that snap either, but it looks like it’s using certbot so you could try certbot renew --dry-run --preferred-challenges http-01 and see if it produces any errors.
I found a certbot executable in /snap/nextcloud/current/bin
So in the /snap/nextcloud/current/bin directory I tried sudo sh certbot renew --dry-run --preferred-challenges http-01
certbot: line 4: import: command not found
certbot: line 5: import: command not found
from: can't read /var/mail/certbot.main
certbot: line 10: syntax error near unexpected token `('
certbot: line 10: ` sys.argv[0] = re.sub(r'(-script\.pyw?|\.exe)?‘, ‘’, sys.argv[0])’
I’m not sure if that’s the error you’re looking for but it looks like a totally different error indeed.
I’m running it on a remote dedicated server rather than a home server bought in. It looks like those instructions are how to log in if you buy an off the shelf home server.