Have you been accepted into the beta tester feature? There would have been an email sent to your Adobe email informing your that you have access to that specific feature.
Are you connected to network? This is required as transcription is currently a cloud service. If you move your mouse over the ! to the left of the Cancel button, you should see a tooltip explaing the error (eg: Unable to connect to cloud service)
Would be great if there was a way to troubleshoot the network error. I'm trying to debug where the request destination is for transcription to make sure my company's firewall isn't blocking it through VPN or locally deployed software.