Sizing Flyback Diode
Your "catch" diode could have instantaneous current equal to your load current, so look for a fast switching diode with a current rating at least 12 amps.  As @Sunnyskyguy points out, your solid state relay (SSR) is undersized for a 12A motor.  Note that the SSR spec labeled "Load Current, DC Motor Starting UL508 FLA @ 40°C [ADC]" gives a value of 5.4 amps.  This means that this SSR is rated to handle the starting current for a motor with FLA (Full Load Amperes) of only 5.4 amps, or less that half of your motor's 12A rating.

Good Luck!