After selecting the Staging Environment Type (Azure Staging Slot) in the Sitecore Azure application, the Add Staging Deployment context menu may be unavailable.
This happens when the Production Environment Type (Azure Production Slot) is already occupied by the Editing or Delivery Farm.
To resolve the described issues, please follow the instructions:
Note: when running the Add Staging Deployment command, the "The database '{0}' already exists." error might appear once in case the databases are shared between the Staging and Production slots. Rerun the command to bypass the error.