Hi,
I understand that you are using an 'entity queue' in your subsytem, and are trying to provide the entity queue parameter 'capacity' as a mask parameter.
You can achieve the same by assigning any variable name, for example 'c' , to the parameter 'Capacity' , inside the block parameters of entity queue. Following that, you may create a mask for your subsystem, and in the 'parameters & Dialog' tab inside the mask editor, you can add the variable 'c'. This will enable you to change the parameter 'Capacity' as per your need, using the interface provided by the mask.
Please refer to this Documentation link to get a better idea on how to create a mask: https://www.mathworks.com/help/simulink/ug/how-to-mask-a-block.html
