After Effects 17.5.1 edit in adobe audition greyed out

You're welcome! I hope it helps make things easier for this project.

I would just do this all in After Effects while I time out the sound effects and the animation. Then I would export a ProRes file, import that into Audition and do my final mix.

However, I just thought of something else that could make this even easier: Audition can natively import and export Premiere Pro project files, and After Effects can export Premiere files. So....you're still going to need to export a final video of your animation, but rather than have to rebuild your SFX timeline for Audition, you can do "File > Export > Export Adobe Premiere Pro Project" and import that into Audition! There might not be any video in Audition, but all of your audio clips will be there in the proper time. If you're working video files or maybe photos in After Effects (I just tried with video) then that should show up in Audition, but native assets like Shape Layers don't show up.

You said it yourself, this workflow is completely new to you, so why apologize for being dense when you're just asking a perfectly good question? There's certainly nothing dense about asking questions in my book!

In terms of Premiere, yes, you could send an entire sequence over to Audition with no audio and add audio there, but that workflow is a one-way trip. Your video will by send over as a flattened Dynamically Linked clip, and you'll be able to build your timeline. Depending on how many effects or the resolution of your video, or how compressed it is, that video might not play back smoothly. Audio effects take a fair amount of processing power as well, so one workflow is to send a finished video with your rough audio mix to Audition, mix there, then finalize then either export a final video from Audition (it can do that now) or export a WAV, which you then import back to Premiere. There's even a third option to send your Audition timeline back to Premiere, but I don't use that as often.

Knowing how slow After Effects can be to cache frames to play back in real-time, you can now see why the same worfklow to send an entire sequence isn't available from AE to AU. You would never be able to play anything back in real-time, so this is why you can only send a single clip to edit.

My typical workflow from AE to AU is to animate to a scratch track, and eventually a rough music track. Then, when my animation is finished, I'll export a ProRes that I import to Audition. I'll use the audio from that ProRes as my placement for the voice-over and music, add any sound effects, then do a full mix and export an final ProRes file. So I actually use Audition as my finishing software ever since they added the ability to export videos and send to Adobe Media Encoder.

Do you have an audio layer selected when you go to select that menu command? It's the same thing for Premiere. In Premiere, though, you can send an entire sequence to Audition via the "Edit > Edit in Adobe Audition > Sequence..." command, but After Effects only allows you to send a specific audio or video clip, not a pre-comp with audio in it.