Thanks!
My suggestion would be that you go back to following the tutorial, but wherever it says
/etc/letsencrypt/live/$(hostname -f)
you would instead write
/etc/letsencrypt/live/on3w-isp.tk
The tutorial assumed that your server would know of its own DNS domain name, but it doesn’t, which isn’t particularly unusual. Therefore, writing it in explicitly should fix the instructions from the tutorial.
When you try again, if any of the commands from the tutorial responds with any message, you can show us that message so we can understand if anything went wrong. But I think it will probably work with the modification that I suggested above.