Hi Rahma Ksouri,
I tried to replicate the scenario and was successfully able to retrieve content of shared OneNote notebook page using users/{Notebook Owner's User Id/ UPN}/onenote/pages/{pageId}/content Please ensure you provide Notes.Read.All permission.
Please check below screenshot:
Please refer the below doc for more details: https://learn.microsoft.com/en-us/graph/onenote-get-content
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