Cannot copy Double-sided

I seem to be having trouble copying double sided documents, I have followed the instructions in the manual to make sure the machine is in copy mode and to press the "duplex/split page" button. Then I get the sub-menu of "2-Sided Copy" and "Page Separation" and then I am to press "enter" on "2-Sided Copy" but when I try to do this a message comes up stating "cannot be combined" and I have no idea how to fix this problem. There is nothing in the manual to help me try and troubleshoot this problem so if you could please help me out it would greatly appreciated.