This is a known issue with changing to 4.12 and using tasks. It can be resolved by simply adding “GameplayTasks” to the PublicDependencyModuleNames section inside your ‘ProjectName’.Build.cs For example:
4.19 still has this issue. but I’m a bit curious. I have another .cpp and .h that use the virtual EBTNodeResult::Type ExecuteTask(UBehaviorTreeComponent& OwnerComp, uint8* NodeMemory) override;