Solved
NAC Radius Not Enable Log
I have already enable radius on fortigate and fortinac and then the connection is successiful.
But when guestusers authenticate from wireless NAC radius log show like as below i have attached 
It seems like you are hitting a known issue (#1129743) which affects new deployments (7.6.2) and WiFi RADIUS authentications. The issue is already identified and a fix will be available in the next release of 7.6.3 scheduled to be delivered at the beginning of next month. For more information you can open a ticket with TAC support.
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