In v9.0 of the DevTest Console, when a user attempts to modify the "Capacity" of a virtual service, they are presented the following error dialog ::
This error occurs because "Performance Mode" isn't enabled in the VSE.
So either the VSE doesn't have a performance license, or as what happened in my case, the VSE does have a performance license, but the VSE isn't configured to run in "Performance Mode".
Details of my particular issue can be found at :: Is Virtual Service Causing Read Timed Out Exception
Ideally, this error message should be much more descriptive.
For example, something as simple as "VSE Performance Mode must be enabled to modify this value" would be much more helpful for users.
The way this error message is now, the user has no idea what the problem is. The user has to go out and search the internet and hope to find someone who might know, like I did.