You must define the inputs and outputs for the symbol to work. You can define the Port and Geometrical outputs as shown below:
[SymbolOutput("StrcuturePort", "StrcuturePort")] // Port Outputs
[SymbolOutput("PinPort", "PinPort")] // Port Outputs
[SymbolOutput("Body", "Body")] // Geometrical Outputs
[SymbolOutput("Pin", "Pin")] // Geometrical Outputs