-Is the server reachable? -> ping from the client device to your FG
-Is the vpn request packets hitting the FG interface? -> sniff at the interface while try connecting (diag sniffer packet INTERFACE 'host SOURCE_PUB_IP')
If not, you need to troubleshoot the client device and local internet.
If both are ok, you need to run ike app debug on the FG like in below cookbook.
That would probably tell you why your FG doesn't accept the vpn request.