When the "No TX" check box is used and the RX line rate and clocking parameters are changed from the default, selecting REFCLK0_Q0 results in an error when the core is generated.
This error is a result of the TX clocking settings still being set and checked against when "No TX" is selected.
To work around this issue, select a line rate and reference clock rate that match the desired RX settings before checking the "No TX" box.
Once this is done, the Wizard will see that the reference clock is correct for both the RX and TX, and allow for the use of REFCLK0_Q0.
This is a known issue for versions 1.4 and earlier and will be corrected in future versions of the core.