Thank you for reaching out.
It could be because Azure AD you created does not have Office 365. Thus it cannot have calendars. /users works because that is an Azure AD endpoint.
If you want to access calendars for your development O365 tenant, register the app in your O365 AAD tenant that you got when you created the development tenant
Follow the steps below -
- You need a Microsoft office 365 account with a subscription, can get this by applying for one on office 365 develop.
- Upon creating one, follow the instructions and add the apps to it.
- Go to Azure Portal and log in with your office 365 account.
- Create an app under AAD -> App registration
- Try to access https://graph.microsoft.com/v1.0/users/{user_id}/calendars.
Hope this helps.