Had the same issue on OS a couple of times. Couldn't add my SK EuroBonus number... always glitched out and said the number was invalid or "did not match the pax name"... Funny enough, if was also impossible to add it via the LH or Swiss sites. Just didn't work.
What actually did work... pulled up the reservation on AirDolomiti (
https://www.airdolomiti.eu/) and added the FFP number there.