It is a bit tricky, but you need to manually add 1 grouping column, and then switch to the flow variables tab. There will be a setting with a 0
as label. Select this to be controlled by your flow variable. (There is a similar here for adding an attachment to an email by flow variable - "Data to report" node generate report with dynamic names using this method)
This slightly strange comes about knime doesn’t have array flow variables
Steve