When building an Azure VM with PowerShell you need to first login using the Login-AzureRmAccount. After that, use the Get-AzureSubscription to list the subscriptions on the account. 

There is one (or two) steps missed between these two commands. Per the MSDN documentation, Get-AzureSubsciption requires the Add-AzureAccount command first, or use the Get-AzurePublishSettingsFile, Import-AzurePublishSettingsFile to install the publish settings file locally.

The correct process is:
Login-AzureRmAccount
Add-AzureAccount
Get-AzureSubscription

The link below lists the requirements for the Get-AzureSubscription command.
https://msdn.microsoft.com/en-us/library/dn790366.aspx