This entry on adding a row to Google Sheets includes screen prints for various options including “Iterate This Task” which I do not see when configuring the Google Sheets AddRows connector. Why does my version not match?
The iterate task toggle option is available in BPM flows. You seem to be using an action flow here.
Here is a quick guide on difference between action flow and BPM flows (also called as workflows).
I hope this answers your question.