SQ stopover award might be showing some phantom availability. Or someone else might have grab the award.

encounter this before for SIN-NRT(stopover)-LAX.

I believe that if the original date (ie) 23/Oct is showing waitlist for MNL-SIN-SFO and when you choose stopover, and change the date of the SIN-SFO (which is showing available), it will encounter error as the availability for that married segment for that particular date is not available. So you can’t make use of a stopover to create the award.

I just try 22/Oct (MNL-SIN) and 25/Oct (SIN-MNL) and it allow me to select both flights. But gotten Same error msg when going to payment.

and I try 23/Oct n 25/Oct, it put me on waitlist instead despite both individual flight available.

best bet is waitlist the segment first.
if not secure SIN-SFO and book MNL-SIN in September under spontaneous escape at 16800 miles. Or pay cash (US125 per pax for Y or US440 for J).