Two separate questions here.

1.- LATAM will only allow to book award flights for roundtrip itineraries. I'm not sure if you can get a CS agent on the phone to book a oneway award (IIRC, most of the time they won't)

2.- Kms redemption for other airlines is quite limited at latam.com. There's been a lot of issues with partner redemption so it may be the case that the service is disabled/inhibited for airlines other than AA, QF and IB.