Safari won't open an account due to unsecure URL
Safari won't open one account due to unsecure connection...only one account affected
"could not establish a secure connection to server"
iMac 21.5″, macOS 12.7
If Safari doesn't work as expected on Mac - Apple Support
If a webpage or link won’t open on Mac - Apple Support
Those two documents include some redundant content, but the cause is always due to one or more of those suggested reasons.
Be advised Safari insists upon strict adherence to current security protocols including proper SSL/TLS certificate implementation (ref, one of many). Many websites are remiss in implementing them properly; allow them to lapse, etc. Other browsers tend to overlook that requirement whereas Safari will not lest there be a worldwide hue and cry that Apple is being irresponsible. If that's the problem you can't fix it. Only the site owner or administrator can do that.
Would you like to learn more? Certificate Trust Store on iPhone, iPad, Mac - Apple Community