Mastering is a text-based Macro or Batch program that tells other programs what to do.
https://www.kozco.com/tech/audacity/Documents/Audiobook-Mastering-Macro.txt
This is my copy of the text file. You can inspect it and then inspect the one you’re using for discrepancies. Depending on how your machine is set up, it may try to hide the .txt part of the filename. I insist all my machines show me the whole filename.
ACX-Check is a Nyquist Program, not a batch file.
My Effect > Limiter is way down at the bottom of the available effects. Effects is presented as two groupings. Limiter is in the second group.
You should try really hard to get Mastering working. It may seem simple, but it does a number of interconnected jobs. It guarantees ACX Peak and RMS (Loudness). That can be really hard to do with separate tools. It also incorporates a blast and rumble filter. Rumble can throw off loudness settings.
This is a Mac, right? When was the last time you shut it down? Close all your apps. No indicators in the menu bar at the bottom.
Apple (upper left) > Shut Down > Are you sure > Yes.
Not Apple > Restart. Watch the screen for odd icons like a spinning daisy. Time it. If you haven’t done this in a while, it may take some time to shut down in an organized manner.
Do you do all the Apple Updates? I wouldn’t do an Apple update during a job. People have had troubles with the last Mac-OS update. Do you have an Intel or M1/M2 machine?
Let’s see. Two others. Are you using iCloud for editing? Stop. Audacity dosn’t like that very much.
Are you filling up the machine? We tell the video people that sudden machine chaos can be caused by the drive filling up. Easy to do when you’re in throes of editing and not paying attention.
If you do this editing all the time, you obviously have a backup machine, right?
Koz