Hi @Mohammad H. Faridi ,
I would suggest that you do a TCP connection test to the VM on HTTP Port 80 and check if connections are going through. If yes, then you would have to reach out to Fortigate team to further investigate why the GUI connection to the appliance isn't working.
If TCP connection on port 80 fails, then please check the NSG rules configured for the VM on subnet as well as NIC level to identify if there are any blocks. Additionally you can also verify the Firewall rules within the VM configuration.
Hope this helps. Do feel free to reach out in case of any additional questions or concerns.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.