This endpoint is a beta endpoint and was not included in the official v1.0 release.
The request method is as follows:
About getting the administrativeUnit in v1.0:https://learn.microsoft.com/en-us/graph/api/administrativeunit-get?view=graph-rest-1.0&tabs=http
About getting administrativeUnit in beta:https://learn.microsoft.com/en-us/graph/api/administrativeunit-delta?view=graph-rest-beta&tabs=http
Hope this helps.
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