I have searched this topic. I have deleted the Google sheet and re-created a much simpler one with text only and without picture/public link.
I have removed the access and re-added it again in Google and Thunkable.
I have re-logged in and refreshed it. The error still remains.
Hi tmcakeed7830, welcome to Thunkable! 
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Can you go into your Google permissions page and see if Thunkable is still listed as having access to your Google Sheets? This error tends to happen when the access for Thunkable from Google has been revoked and you’ll likely just need to reauthorize Thunkable.
https://security.google.com/settings/security/permissions
Hello! It seems I got same error. I noticed one day my button navigation to some pages doesn’t work and also the fetching data problem. Reauthorization doesn’t seem to help.