Answer
A BizAPI needs an actual Handler, and Handlers are modeled and developed inside a .serverpackage file, not directly inside the BizAPI itself — this error means Developer Studio can't find the relevant .serverpackage file (containing the handler you're trying to reference) in your workspace yet. Model and develop the handler in its own .serverpackage file first (see the Services Layer Development and Working with Server Packages documentation), then reference that handler from your BizAPI, rather than trying to build the handler logic directly inside the BizAPI definition.