[Mashiro]

@sagher

The error / warning message is pretty clear (at least in my humble opinion).

The installer tries to create a Let's Encrypt TLS certificate for your server based on the hostname of your server. The hostname of or your server is however a hostname that doesn't exist as a real domain or if it exists it is not pointed to the IP address of your VPS.

The best way to fix this would be to change the hostname of your VPS to a domain that you own and that is pointed to the IP address of your VPS. Or otherwise accept that a self signed TLS certificate will be used.