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After clicking Enable Service, a dialogue with a device code and a link will be displayed. Open the link in a new browser windows or tab and enter the device code. The device code is only valid for a limited time. A counter is showing how much time is remaining, and when the timer reaches zero, the user will not be able to proceed with enabling the service. If that happens, close the dialogue and start again.
The user will be prompted to authenticate with a Microsoft Account. This Account needs to have the Global Administrator role. For more information about this role, see this Microsoft article.
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