Had a similar problem. Fixed it by defining the ARRAY_COUNT back to what it was after the steam api include. Like this in my header file…
#pragma once
#include "Engine/GameInstance.h"
#include "OnlineSessionInterface.h"
#include "steam/steam_api.h"
#define ARRAY_COUNT( array ) (sizeof(ArrayCountHelper(array)) - 1)
#include "DSSGameInstanceBase.generated.h"