I unwittingly used all of our remaining ArcGIS Online credits (5 months worth) using the World Geocode Service in ArcMap 10.3.1. I didn't think it had worked correctly so did it a number of times. I even tried doing it in ArcGIS 10.6 and got the same error message. The error message should also be changed from 'There was an error trying to process this table. ArcGIS Online subscription is required for geocoding the table of addresses' to "You do not have enough credits to do this task. Check your remaining credits". It should also tell you how many credits the geocoding is going to use prior to submitting the task especially when it will use all remaining credits of the organisation.