How to use Transparent Proxy and HTTPS traffic

You cannot use squid to access HTTPS with the transparent mode enabled.

What you could do is adding a rule to the firewall to enable the access to HTTPS.

Create a service for HTTPS traffic whose destination port is TCP/443 through Services. Then, go to Firewall --> Packet Filter --> Filtering rules for internal networks and set a rule to allow the service HTTPS being used by your internal hosts.

Save changes. After that your users should be able to access HTTPS sites.