This started to happen due to changes in the mlxtend library version 0.23.2. It is still unclear whether it is a bug or a feature (open issue). So there are two options to fix that error:

1. Downgrade mlxtend package to version 0.23.1 (if we assume it is a bug) or
2. Add add num_itemsets parameter: rules = association_rules(frequent_items, num_itemsets=len(group_df), metric='confidence',min_threshold=0.7, num_itemsets=2). Here I set the parameter to len(group_df) since docstring is saying it should be to "Number of transactions in original input data".