Admin Consent for an App

Please go to the website 

https://login.microsoftonline.com/{Tenant-Name}/adminconsent?client_id={Client-ID}&state=12345&redirect_uri={Redirect-URI} 

and login the application by your directory global administrator account one time. The permission will delegate to the end user.

More information – Admin Consent

https://blogs.msdn.microsoft.com/tsmatsuz/2016/10/07/application-permission-with-v2-endpoint-and-microsoft-graph