Unable to add Nginx extension set to VM in sandbox

In the exercise "Create an Azure Virtual Machine" I created the VM as instructed, but when I use the

   az vm create \  
     --resource-group learn-xxxxxxxxxxx \  
     --name my-vm \  
     --image UbuntuLTS \  
     --admin-username azureuser \  
     --generate-ssh-keys  

command I get the error:

   (AuthorizationFailed) The client 'xxxxxxxx' with object id 'xxxxxx' does not have authorization to perform action 'Microsoft.Compute/virtualMachines/read' over scope '/subscriptions/xxxxxxxxx/resourceGroups/learn-xxxxxxxxx-cb152d78be0e/providers/Microsoft.Compute/virtualMachines/my-vm' or the scope is invalid. If access was recently granted, please refresh your credentials.  
   Code: AuthorizationFailed  

Any idea what I'm doing wrong?