It’s impossible to “transfer” the content of a MIDI track to an Audio track. If I understand you correctly, you want to make an Audio version of your Instrument Track. One way of doing this is exporting the track as an audio-file and then re-import it to the arrangement on a new Audio Track. Just drag and drop the audio-file from the desktop to your arrangement, and a new audio-track will be created automatically.
Just remember to mute the Instrument Track, after you’ve imported the audio-file. Otherwise, the track will play back twice!
Another option is to “freeze” the Instrument Track, but I’m not sure that Cubase AI has this option. When you “Freeze” the Track, Cubase does all the exporting and importing for you (so to say). The best thing with Freezing is that you can easily “Un-Freeze” the track, edit your MIDI data and then “Freeze” the track again.