If you are seeing an error message when trying to authorize OneDrive for Business, try the following:
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Start here:
​https://developer.microsoft.com/en-us/graph/graph-explorer
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Sign in to Microsoft's Graph Explorer test tool using the blue button at left.
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At the Permissions prompt click 'Accept' to continue (No need to consent on behalf of Org).
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Then find OneDrive on the left side scroll panel and expand its children, click on 'GET all the items in my drive'.

Then click Modify Permissions (top center) and click on the blue Consent button to right of Files.ReadWrite.
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Lastly, again click on 'GET all the items in my drive' at left.

If you do not see a green box with OK - 200 then something is wrong with your Microsoft 365 account. Either your IT admin has restricted 3rd party app access, your OneDrive for Business is empty, or there's an unresolved billing issue.
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If this is the case, no 3rd party app, including Flatly, will be able to connect to your OneDrive for Business.