As far as I can tell, the issue is that you're trying to modify the I/O for one of the variants in a variant subsystem. That means each variant in that subsystem must use a subset of the signals from the parent. For example, if the variant subsystem has inputs A, B and C, then each of the variants must use only those signals. If you want to add a 4th input (D), then you've created an incompatible variant. This is what your error message is telling you.
You have 2 options:
- Update the variant subsystem to include the new signals you want in your individual variant. This will require you updating the model level above this to feed in the new signal.
- Get rid of the variant subsystem and just put in a new subsystem like Ashish recommended. Think of it like hard coding your subsystem I/O. It's simpler, but more rigid in its design.
For more info on variant subsystems, check out this example.