Thanks for letting us know your user experience. According to your case description, I do understand how frustrated you are now.
When I received the case, I searched a lot of documents did a lot of researches. And I feel regretful to inform you that it is not possible to modify the content of password-protected Excel files using the Graph API?
And I noticed that some end users have also proposed the same request, it is highly recommended that you can vote this ticket. Many features of our current products are designed and upgraded based on customers’ feedback. With requirements like this increase, the problem may well be released in the future. Thanks for your understanding.
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