Are you getting this error message on all On Demand content you attempt to view? What troubleshooting steps have you performed? It's always a good idea to check that your connections are tight and unplugging the box to reboot it fixes many common issues. Please also let me know what box you're using.
Along those same lines, I am getting a message that I have to buy TV shows... seems like mostly from CBS. An example is "Blue Bloods." However, another CBS show, NCIS:LA gives me the option of CBS which is free and CBS Films which says I have to buy. Blue Bloods doesn't have that option.