I spent lot of time trying to get Google Sheet API set up for forms.
I finally got the authorisation code from Google but i had to proceed a bit differently compared to your documentation :
www.balbooa.com/joomla-forms-documentati...e-sheets-integration
At step n°7 : i did not publish as suggested but stayed in TEST mode, then i added my google account email.
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I also found that at your post here :
support.balbooa.com/forum/joomla-forms/1...sheets-problem#73374
Step 1 does NOT get working result but if we go only with Step 2 while keeping OAuth in TEST mode : it works.
Proceeding this way get Google to accept Forms app and when Forms tries to authenticate Client ID and Client Secret, it works.
Then, this leads to get the Authentication code afterwards.
Hope this post would help others.