I thought I'd give it another shot this morning. I pulled up a better return flight on DL.com for 25K + $40 and wanted to see what the fees would be for the one-way TUL-CUN on their site. When I choose one-way, the option to use miles is grayed out. The only way I can get the option to use miles is to choose a round-trip, which ultimately still gives me error message 150.

So, new question: can I not book a one-way award ticket on NWA.com?