If that does not help, it is probably Gatekeeper interference, assuming you are on macOS 10.12.1 or later. Open /Applications/Utilities/Terminal.app. Copy the below text then paste it into the terminal:
would this fix also work with the missing ACX Check plugin? It is working on my MacBook Air (mid 2013) running El Capitan 10.11.6. However, it is missing on my MacBook Pro (mid 2015) running Mac OS Sierra 10.12.3. I can see it in the plugins folder, but it does not appear on the list of Effects inside Audacity.
The version of Audacity on the 2013 MacBook Air is 2.0.5. The version on the 2015 MacBook Pro is 2.1.2. I don’t dare update Audacity on the MacBook Air. I did install an older version of Audacity on the MacBook Pro (version 2.0.6 was the closest I could get), but the problem with ACX Check missing persisted.
I am using a Desktop Mac OSX EL Capitan version 10.11.6
I am currently using Audacity 2.1.3
The problem is suddenly I am unable to record in the .mp4 format (the only format my
computer is set for AND that I prefer).
It switches it to MP4 and I have no idea how to switch it back to .mp4
Now, if it proves necessary to update the Audacity version that I am using, WHICH
newer version is COMPATIBLE WITH MY MAC.
This forum is the correct place for support, but if you tag your posts onto the end of ancient topics they are not likely to be seen by many people. It’s best to start a new topic for each new question.