@TrnMinhTrng-3973, Thank you for reaching out. The following graph api "https://graph.microsoft.com/v1.0/users" would list all the users in your tenant. To use this API the minimum level of required permissions are:
- Delegated Permissions: User.ReadBasic.All
- Application Permissions: User.Read.All
Make sure that you are logged into GraphExplorer app using a user account of that tenant and also make sure the following permission delegated permission "User.ReadBasic.All" is available so that the list of users can be fetched from AAD.
Hope this helps.
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