Open setting and search system apps in search area.Select system apps setting and then select call setting. Select incoming calls and enables the flash on incoming call.
Setting-system apps-system apps setting-call setting-incoming calls-enable flash on incoming call
Step-2
Download flash alerts 2 app from Google Play Store. Open it and give it to all permissions.
Google Play store-flash alerts 2-enable all permissions.
Hope this post helps you !!