Build failed happen when convert subsystem to model reference

I have 1 model with top level is a subsystem, i can success build it. But, when i converted that subsystem to a model reference, build failed happen with this information;

Error: Invalid data scope setting 'File' for data object 'ABC' when code is generated for model 'ABC'. The data object is either used in more than one file in the generated code, has no readers or writers, or is a signal that is connected to a root input/output port. Modify the data scope of the data object to either 'Auto' or 'Exported'.