UA isn’t a general travel agent and won’t book all revenue flights on even partner carriers. They should allow for a redemption on any *A flight, presuming there is saver availability (save for the airlines that aren’t working right now, of which AC is fine). If there is no ‘fixed mileage’ (equivalent to saver on UA) availability on the AC (really, Rouge) segment, then you won’t see anything come up on UAs site.

One thing to check is is if you are looking on the right days. YYZ-GND doesn’t appear to be a daily flight. Also poorly timed for connections. I can’t see the app allowing anything revenue on what I checked, but do see Awards on days where Aeroplan is showing fixed mielage availavility. Do note that there flight leaves fairly early from YYZ, depending on the day I see anywhere between 6:30 and 9:30am, so you’ll have to arrive the night before. The return arrives YYZ in mid-evening, between 7:30 and 9:00 pm depending on day so connecting onward same-day is limited to non-existent.

Also note this flight is operated by Rouge. So just know what you’re getting...not even regular AC standards.