Option -s , enables SSL verification and is on by default.

Testing within environments where there are self signed certs, is causing the tool to fail.

Debian (Kali) Certificate for this server is in ca-certificates folder and added. Error message is as follows, server name changed, etc.

[16:49:44] INFO Checking whether host example.com:443 is available tester_utils.py:41
ERROR Unable to connect to host example.com:443 due to error: SSLCertVerificationError(1, tester_utils.py:49
'[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self-signed certificate in certificate
chain (_ssl.c:1006)')
ERROR Stopping tests due to unavailibility of host: example.com:443

Also, it appears your error message has the incorrect spelling of unavailability. :)