UMG is Slate based (i susspect you talking about UMG), so you will need to include “Slate” and “SlateCore” (or just adding UMG module it enouth?) in to dependencies in you build script of your module. It also means, it is cleaner to create your own USpinBox instead of deriving it, because you UMG widgets operates Slate widget (in this case SSpinBox), so instead conflicting with existing code in USpinBox, it better to operate SSpinBox from the clean UMG widget. So look up how USpinBox is constructed and build UIntegerSpinBox which will operate SSpinBox with integer
https://github.com/EpicGames/UnrealEngine/blob/8a80b5541f69a79abf5855668f39e1d643717600/Engine/Source/Runtime/UMG/Private/Components/SpinBox.cpp
https://github.com/EpicGames/UnrealEngine/blob/8a80b5541f69a79abf5855668f39e1d643717600/Engine/Source/Runtime/UMG/Public/Components/SpinBox.h
I don’t know if it will be too complex for you, but it a worth a try, you can learn something from this challenge (like using Slate)