I tried to book an award ticket from NRT to TPE, then from TPE to NGO via ICN (TPE-ICN on TG, then ICN-NGO on NH).

The UA CSR went ahead and managed to book everything for me and even issued me a confirmation number. Then just before she hang up, she told me she needed to get a reply from TG (she got OK from NH already), then after being put on hold for 5 minutes, she told me that the routing I asked is not legal, and the reservation could not be done.

Can anyone enlighten me on this? Why is this an illegal routing?