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I would like to do a test but do not have the same test environment.
Based on my research, I find a link "What do I need in order to use Grammarly?".
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The error message indicates that Office Store Add-ins might be blocked by the Administrator.
You could try the methods given by Dihao Guo MSFT in this thread "Office Add-ins prevented by 365".
Hope the information is helpful to you.
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