Hello Pablo,
I will first summarize what I understand of your query. You seem to have created a simple azure AD free tenant. And created a user within the same tenant . After this you tried to register an application and provided consent for it using your global admin ID . And now you tried accessing you calendar Information for the user account.
If you were trying to use the user account pablo.martinez7908@Stuff .com , then you would not be able to get the calendar for this user because in Azure AD the actual username for this object would be different and you would never be able to provision a mailbox for this . Its an account actually residing in Microsoft Account system (liveID system) rather than azure AD . Azure AD holds just the reference for it . So the actual username for this account would be pablo.martinez7908_gmail.com#EXT#@.onmicrosoft.com which you may not be using in postman to get the token.
If you have created a user with user@.onmicrosoft.com then you would not get calender information for this user because the user does not have the calendar service provisioned. You require a exchange online license assigned to the user in order for the user to be any calendar information present.
You mentioned that when you used the user created in AD (I am assuming on-premise AD) , the token was generated and you get a different error while accessing https://graph.microsoft.com/v1.0/me/calendars . I am assuming that the synced user from AD to Azure AD does not have a exchange online account associated with them and that's why you do not get calendar information.
I hope the above clarifies your query.
Now coming to your problem which you have mentioned earlier that you want to connect your calendar with a windows service in .net C# code. I am not sure if we have a ready made sample for the same but you can check this one . If you want to test this I would suggest you to go to Office dev Center https://developer.microsoft.com/en-us/office sign up for developer tenant which is free for limited period and will also provide you with Office 365 E3 dev licenses. You can use this license to provision the calendar of any test user by assigning test E3 dev license in your O365 Dev environment . Once this is done the calendar provisioning will happen. Also you would need to provide your application registered in Azure AD calendar.read permission in Graph API
I hope the answer helps . If the information provided in this answer helps you , please feel free to mark it as answer so that it is helpful to other members of the community. If you have any other queries related to this or if you feel we have misunderstood your query. Please do provide dtails in comments and we will look into this.
Thank you.