Hello Seunghoon. The GTFS specification only started to allow blank values for stop_lat and stop_lon fairly recently when it adopted support for platforms.txt. The Add GTFS to a Network Dataset toolbox you are referring to is now deprecated and has not been updated since that change. That's why you're getting an error.
In general, I do not recommend using the old Add GTFS to a Network Dataset toolbox anymore. Instead, you should use the newer, better tools available out-of-the box in ArcGIS Pro. Learn more here.
However, switching to Pro will not actually solve this problem until ArcGIS Pro 2.8 is released (later this year). For now, you will need to manually update your GTFS data. Since the blank stop_lat and stop_lon rows probably represent non-geographic location_type values of 3 or 4, I suggest you just delete the rows. They will not be used in the analysis anyway.