OK so that is unicast, not multicast; should make troubleshooting a lot easier.
FIrst, let's try and see if we can eliminate the FortiGate as the issue here: Try adding the printer using that method from a device that is on the same VLAN as the printer, please. If it works then we can look at the FortiGate.
OK then we have to figure out what the firewall is doing with the traffic. Can you please set up a flow trace. Set the source address to that of the endpoint you are trying to add the printer to and the destination address of the printer.
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