What error message are you getting? That may shed some light on the underlying problem -- it could be a case of phantom/cached availability (e.g. UA thinks that x number of seats in Y (or E or N or ...) class are available on the partner flight so it's showing you a fare based on that availability and fare rule, but since the last snapshot of availability was taken those were sold or otherwise made unavailable, so when UA goes to actually book (NN1 / SS1) the segment Air Dolomiti's GDS, instead of returning HK1 (confirming the seat is held) it replies with UN/US -- the GDS version of "what are you talking about? I have no Y/E... seats to sell you" and UA bombs out. )