The tele is an AF-S TC-17EII but it will not mount to the sigma. I can mount the Sigma APO Tele 2x EXDG but would rather not use an f stop f13
Am I doing something wrong?
No you are not. Nikon uses a lockout tab to prevent mounting their TC's on incompatible lenses. Sigma makes a 1.4x TC that is FULLY compatible with your lens.