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Hi @Thomas Chan
Thank you for posting your question in the Microsoft Q&A forum.
From the message you provided, it seems like the account trying to access SharePoint is an external account, which mean the user is not a member of your tenant.
So, they will not have permission to access your SharePoint sites and contents.
Error AADSTS50020 - User account from identity provider does not exist in tenant | Microsoft Learn
You'll need to add that user as guest user in your tenant.
Please follow the steps here:
Quickstart: Add a guest user and send an invitation - Microsoft Entra External ID | Microsoft Learn
This will grant the permission so that external user can access internal data.
I hope this information helps.
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