This usually happens when your Google Sheet does not contain any header row.
 To fix this:

1. Go to your Header Settings inside the plugin.
2. Make a small change and save the settings again.

This will automatically regenerate the required headers and the sync/update process will work correctly.

It is not recommended to clear the sheet manually.

Please use the Clear Spreadsheet button provided in the plugin.

If you absolutely must clear the sheet manually, make sure to keep the header row intact. The plugin depends on the header row for mapping and syncing.

This error occurs when your sheet doesn’t have enough columns.

Solution:

Add a few blank columns to the right of your last column, then try saving the settings again. The error will be resolved once the sheet has enough column space.

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Google might display a screen that says “This app isn’t verified. This app hasn’t been verified by Google yet. Only proceed if you know and trust the developer.” There’s no need to worry, though — the message is only referring to the app that you just created in your own Google account (so you are actually the developer in this case).

To bypass this message, you’ll need click Advanced and then Proceed.

1. Make sure you have allowed all the checkboxes when generating the authentication code.
2. Make sure you publish the app.

There are following ways to resolved this error:

1. Revoke token at Google API Settings Page.
2. Check the Google Drive API must be Enable

You can synchronize existing documents by clicking the ‘Click to Sync’ button on the Document Settings page. However, this action will not update the data already present in the sheet.

Please add 1000 rows to your sheets. You will find this option at the bottom of the sheet. Please check the below screenshot.

If the header row still doesn’t update:

1. Go to Header Settings.
2. Add or remove a header field (any small change).
3. Save the settings again.

This forces the plugin to rebuild and refresh the header row in the sheet.