You need to make sure there's availability. Just because a flight exists on an airline you could theoretically use miles on, you can't use miles unless the airline publishes availabilty in a way that's avialable to the airline you have miles with.
LATAM is notorious for having problems at times making their inventory available to other oneworld airlines. And on some routes that are expensive (because demand outstrips supply), they may not release much in the way of award seats anyway. (I know these things from following the LATAM forum for several years and the AA forum for many many years.)