As DVDDoug above, you have two problems. The combination of RMS (loudness) and Peak is out of range with each other and the Noise is too low. The only way you can get noise that quiet is to mess with it. ACX hates messing with it.
Start with the raw reading (no processing at all) and apply Audiobook Mastering 4.
https://forum.audacityteam.org/t/audiobook-mastering-version-4/45908/1
It’s only three tools, but you have to download one of them. You already have ACX Check.
There’s a note about ACX Check. You can fake it out. You have to have at least 3/4 second of pure background sound (Room Tone) for it to work right. This is a little posting from a separate page.
http://www.kozco.com/tech/audacity/TestClip/HoldYourBreath.html
If you don’t have access to the original raw reading, that’s mistake number three. It’s a New User mistake to patch, adjust and filter the original reading on the way to a completed audiobook posting. Instead, at the end of a raw reading, File > Export a WAV (Microsoft) 16-bit sound file. Save that somewhere safe. You should never have to read something again because of a machine or Audacity crash.
If you haven’t closed Audacity yet, you can Edit > UNDO to get back to the original reading. If you have closed Audacity, that’s the end of the world because we can’t take effects, filters or processing out later.
ACX requires all your chapters to match, so you can’t custom patch each reading.
Let us know where you get stuck.
Koz