The minimum refclk frequency for an Arria® 10 device transceiver fPLL is 50MHz.
The Quartus® Prime will not allow the use of a reference clock below 50MHz other than when used with the HDMI Design Example.
For the HDMI Design Example only, a 25MHz refclk is used. Refer to the Intel Arria® 10 HDMI IP Core Design Example User Guide to generate the HDMI Design Example.