Commercial Insurance and Medicaid/Medicare

I just got off the phone with a commercial insurance plan as I was checking on denied claims for a patient. This patient has MA has a secondary. The commercial primary is denying due to the COB box on the electronic claim was checked yes. Commercial insurance rep told me that if a patient has Medicaid/Medicare that box can't be checked yes. Can someone please help explain this to me and why? Where is that box in Procentive? I have numerous denied claims for numerous patients with this same issue?

THANK YOU!