Running panos 10.1.8 on vm-300.
A blocked https page returns "This site cannot be reached" rather than the response page.
Is this because ssl decryption is not enabled and is it required to get the block page notification for a https url which are supposed to be blocked via url filtering ?
Correct, the response page won't be serviced unless you're decrypting the traffic. You can override this setting by running the following command without setting up decryption.
set deviceconfig setting ssl-decrypt url-proxy yes
Keep in mind that this will only work if you aren't in a vwire deployment, ssl decryption is an absolute requirement for that configuration. Keep in mind that your clients will receive an invalid certificate warning unless you take the time to setup a proper Forward Trust certificate to allow the response page to actually be trusted by the clients.