The error message is as clear as it gets. Open the example model of the Variant Subsystem block to learn its proper use.
The direct way to resolve your problem could be running the below codes in MATLAB.
MyChoice=1;
variant_generation=Simulink.Variant;
variant_generation.Condition='MyChoice==1';
variant_simulation=Simulink.Variant;
variant_simulation.Condition='MyChoice==2';
