I believe Synology uses a graphical interface for this, as opposed to a command line. That being said, I’ve never used Synology and thus I’m of no additional help.
Is that domain name currently pointed at your Synology device? It resolves to 216.239.141.7 and appears to return a web page that might not be your Synology device but might be some other web server.
My understanding is that the Synology Let’s Encrypt client will only obtain certificates successfully for names that are currently pointed at the Synology device where you’re requesting the certificate.
Can I obtain a Let’s Encrypt certificate without the Synology server? I only need it for a Google Maps page on my website (new https map requirement from Google).
The expected path in this case is to run a Let’s Encrypt client application on your web server (which is then a machine entirely separate from the DSM and that uses entirely different software). What kind of server environment do you have on the web server?