Graph API - Insufficient privileges to complete the operation

Please refer to below steps :

1. From your screenshot , seems you grant Read and write directory data application permission for Windows Azure Active Directory(azure ad graph api) . Since you are using microsoft graph (https://graph.microsoft.com/) , you need to grant application permission for Microsoft Graph :

2. Since you are admin in your AAD, You could grant permission for users in organization by click Grant permission button shown in above screenshot .

3. Then you could use your code (client credential flow to get the token) and query users information . If you check the claims in access token issued by azure ad , you could find Directory.Read.All permission in roles claim .

For me the key to solve this problem was hint:

To use the Graph API with your B2C tenant, you will need to register a dedicated application by using the generic App Registrations menu (All Services and there it is by default not Favourite starred) in the Azure Portal, NOT Azure AD B2C's Applications menu. You can't reuse the already-existing B2C applications that you registered in the Azure AD B2C's Applications menu.

Find more on page AD B2C API access demo

Make sure click "Grant Permissions" and than Yes for all users accounts.