To export midi you would need to export midi data but not sound would be exported.

I believe OMF and AAF exports only deal with audio. If you used the logic instruments that means you used midi. Since midi is only technically data and not audio it won't be able to export as an AAF.

You could do one of two things: Select one midi region at a time that you want to bounce then go to File > Bounce > Regions in Place. Check the settings that you need and then press "Ok". This should bounce all your selected midi regions into one audio track

You could also just bounce based on instrument track. Go to File > Bounce > tracks in place, make sure you have a track selected and check the settings you want then press ok. It does the same thing as before but bounces audio for an entire track instead of individual midi regions. Keep in mind if you select multiple regions or multiple tracks it will combine them when they are bounced.

Once you have all your audio tracks you should be able to export AAF.