We have started to use tenants so that we can deploy arm templates to different subscriptions from a single project. But seem unable to selected a Azure service principles from the azure account drop down on the tenant variables screen.
I was able to replicate this issue. It seems this is a bug in the portal UI.
I have raised an issue which we will resolve ASAP (hopefully in the next few days).
In the meantime, as a workaround, you should be able to change the ‘Control type’ of the variable template to ‘Single-line text box’ and enter the name of the account as the value. As long as you use this variable as the value of one the Azure Account fields (e.g. on the Azure Web App step) it will correctly resolve at deployment time.
I hope that helps. We sincerely apologize for any inconvenience.