A workaround I found to print a receipt for an award trip, if you also have other trips, which are non-award, in your list of reservations.
- Go to a "normal", non-award trip, which does have a Print trip and receipt option.
- Click, to get the receipt. This opens a new tab (or new window, depending on your browser). Do not close this new tab!
- Go back to your trips and select the award trip whose receipt you need. When it loads, switch to the still open receipt of the previous trip and reload that window. It will now load the receipt of the award trip.
Neat! Similar to the multi-tab hack for changing the name of a PNR. Seems to me, though, that the browser-makers are working on clamping down on cross-tab/window activity for security reasons, so these may stop working in the near future.