It appears the infant is causing the issue - Tried a dummy booking without infant for same
flight/date, and it is showing seating options.

Going to try and call and see if I can get/buy a seat assignment that way. Don’t know for sure, but wonder if there are limited rows in the ATR where one can sit with a lap infant - I know for example on the Embraer 145s, there is this issue with only 4 or 5 rows being able to accommodate an infant as these are the only rows with an extra oxygen mask in case of emergency.