Problem
I recently ran into an error while deploying an update to an existing cloud service in Microsoft Azure. The change I made was to include a element in my Service Configuration (*.cscfg) file to include this service into a virtual network's subnet (you can read more about this process here).
The error message is clear, but not necessarily obvious. You cannot make changes to virtual network settings against a cloud service during a deployment update or upgrade. So what can you do? You can make changes during a full deployment.
Fix
Refer to the Publish Windows Azure Application Wizard on MSDN, specifically #7 in Configuration Your Settings. 
When 'Deployment Update' is checked, only the changes since your last deployment will be pushed out to update your services. This is not possible when changing the virtual network settings of the service. To enable full deployments, you have two options:
- Uncheck the 'Deployment update' checkbox
- Click 'Settings' on the 'Deployment update' item, and select 'If deployment can't be updated, do a full deployment'.
Please note that performing a full deployment will reset the VIP of your cloud service. You can learn more about Retain a Constant Virtual IP Address for a Cloud Service