If you have an application registration setup in Azure for custom app access to your Office 365 tenancy which requires administrator approved permissions then you will first need approval from an administrator at least once in order to activate these permissions.
To do this the easiest way is to create a link which can be sent to a company administrator to approve and essentially activate the permissions.
To do this create the link with the following format:
https://login.microsoftonline.com/TENANCYID/oauth2/authorize?client_id=APPIDNUMBER&response_type=code&redirect_uri=https://TENANCY.sharepoint.com&resource=https://TENANCY.sharepoint.com&prompt=admin_consent
When the administrator clicks on the link they will be prompted to login with an admin account before then being prompted to approve the new permissions.
Once approved you will have access to all of the permissions granted to the application within that resource
