SQ are notorious for poor seat selection. Even with *A Gold I can only choose from a few coughpy seats. I had a paid J flight with VA code share and was unable to choose seats. This is what happens when you're in a faux alliance. VA need to hurry up and join *A already, I have moved all my flying to maintain my *A gold instead of my VA platinum. Get much better service on Thai than I ever did with SQ
I will give you a tip, if you can't get in online to select your seat, try downloading the SQ app to your phone. But don't expect a large amount of seats to be available for selection. Failing that you can email them and they put a request in for you, with the usual "we can't guarantee"