We need to configure the trigger (VIP real server disabled) , action (email).
Need to make sure the email settings are correct - ref : https://help.fortinet.com/fos60hlp/60/Content/FortiOS/fortigate-system-administration/Monitoring/Ale...
Also, do you see the "VIP real server disabled" event under logs >system events?
For testing the email alert, we may use some other logs like login/logut events as trigger and then change the event to the LB log.
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