why is find friends not working on IOS 13

iOS 13 combined the Find My Friends and Find My iPhone apps into a single app with the name "Find My."

The new app should already be on your phone. If you don't see it, it may be in the same folder where your Find My iPhone app was before the upgrade. You can use Search to find it.

To add to the information that FoxFifth provided, when you go to the "Find My" app, look along the bottom of the screen and if you tap on People, it will show like Find My Friends, and if you tap on Devices, it will appear like Find My iPhone.