On trying to publish an application from the 'Add Applications' wizard in a Delivery group, we receive the error : There are no powered on machine that can be browsed

Make sure that VDA is registered on the machine and the machine is registered. 
Try restarting Citrix Desktop Services on the VDA Server(server hosting the application)
If there are issues with registering VDA, please follow the steps mentioned in the following article to troubleshoot the issue : 

Problem Cause

We are unable to populate the applications installed on the server as it is unregistered 

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