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In what manner are you trying to print?
From what program, and from a Windows PC, or an Apple computer?
11 x 17 paper is supported. Have you defined it as a Custom Paper Size ? if so, be sure you haven't got the dimensions reversed, so that you have defined it as 17 x 11, which won't work since you cannot put 17 inch paper in the printer's tray sideways.
Let me know more, please. E.g., how the system refuses 11 x 17 paper size. Thanks.