you could have convoyed Trieste-Ankara if you had used 2 fleets, one in Adr or Ion and one in Aeg.
in fact, your fleet in Ion could theoretically move to Tri (let's say board the army in Tri) but could not move to Ank (3 moves away!) and so it couldn't convoy there.
you could have successfully convoyed to Smy, Syr, Con, Bul, Gre, Tun, Alb, Ven, Apu, Tus, Rom.
the system gave it as an option since there was another fleet in Aeg which could have theoretically helped you.