Seems like you have to trick the validation
When I booked a Singapore Airlines flight from SFO to SIN there was a United - MileagePlus option. However after I entered my account I get this error:
"Please enter a valid frequent flyer number."
Turns out the only way I could "fix" this was to:
- Check the consent checkbox
- Pick United - MileagePlus dropdown option
- Enter the United number
- IMMEDIATELY (do not click anywhere else) click on "UPDATE" button on the bottom.