Azure powershell Select-AzureSubscription - error: the subscription name doesn't exist

If you are using Resource Manager, the correct cmdlet is:

Get-AzureRmSubscription –SubscriptionName "<name>" | Select-AzureRmSubscription

or just use -SubscriptionId instead of -SubscriptionName.

Select-AzureSubscription is to be used to manage classic resources. To make it work you also have to add classic credentials using:

Add-AzureAccount