I have been trying to attach my United MileagePlus number to a THY reservation for a couple of months, but always get that this number is not valid (when it is).
I called their reservation center, asked them to enter my M+#, which was confirmed, but it is not attached to my reservation.
I also send messages to their Help center, but never got a helpful answer. All replies were deflecting the issue, that does not seem complicated.
Are they not willing to fix the problem or is it done on purpose?