How to download iPadOS 13 simulator for Xcode?
Xcode 11.3 supports developing apps for iOS 13.3, iPadOS 13.3, tvOS 13.3, watchOS 6.1, and macOS Catalina 10.15.2.


So make sure you have Xcode 11.3, which is available for 


  a Mac running macOS Mojave 10.14.4 or later.


https://developer.apple.com/documentation/xcode_release_notes/xcode_11_3_release_notes?language=objc

Also, make a simulator if not already there in Window menu →  Devices and Simulators window. Choose device and it should show available iOS/iPadOS. If not present, hit "Download more simulator runtimes" and see if it shows up in the list. 

https://help.apple.com/xcode/mac/current/#/devf225e58da
https://help.apple.com/xcode/mac/current/#/deva7379ae35