For some reason, you can’t modify it in the GUI, but you can modify the keyword ‘BC_JJ_PS’ if you open up the .mfx file in editor. 0 to turn it off, 1 to turn it on. After turning it on, you will be able to modify specularity and restitution coefficients.
Someone had a similar issue to which I responded here:
@onlyjus Don’t know if I’ve found a bug, or a workaround to a feature I don’t understand how to use. Even when I activate pure granular flow, the following still appears, where I am unable to toggle JJ on, but can modify the JJ-type. Only able to do so after changing internally in editor…