I did regenerate the Google token. No luck.
I may have added Flamingo plugin after installing NS Google Sheets Connector. Sorry, I forget which of the two I added first. I’m pretty sure yours first, then Flamingo.
Other changes: I removed two “fields” from the Contact 7 form linked to my google sheet. Other edits to language were made in the form. I deleted the columns in the google sheet that were associated with the deleted fields in the form. And I added notes to records collected in my sheet – I used columns further to the right. They were ones not in use (empty).
I also installed Yoast SEO plugin after getting your plugin up and running.