Try this
UPlayerController* PlayerController = ...
PlayerController->DeprojectScreenPositionToWorld(ScreenX, ScreenY, OutWorldLocation, OutWorldDirection);
More info on: >DeprojectScreenPositionToWorld<
OutWorldLocation and OutWorldDirection is your ray that you can shoot from the scene. You can check if it hits a plane with this API
FPlane PlaneToHit = FPlane(FVector(0, 0, 0), FVector(-1, 0, 0));
FVector WorldHitPosition = FMath::RayPlaneIntersection(OutWorldLocation, OutWorldDirection, PlaneToHit);