Please make sure you have set correct site url (including https if you have ssl ) from WP Admin => Settings => General
To get Client ID & Client Secret you will need to create a new Project with your google account.
-Go to Google Console & login with your gmail account.
-Click on Credentials menu on left & then click on Create a project.
-Add a name for your Project, select radio option below that and then click Create
– Enable the drive & sheet api for your project.
-Select Drive API & Sheet API
-Now click on Credentials on left sidebar & then Click on Create credentials and select OAuth Client ID
-Give your Product a Name e.g “Formidable Integration” and the click Save button at bottom.
– Next Select Application type as Web application and add your site url as Authorized redirect url ( Important : Url should be with slash at the end e.g http://mysite.com/ )
Click Create button at bottom.
-Next Copy the Client ID & Client Secret and paste it on Ninja Forms GoogleSpreadsheet Addon Settings