I searched the forum and found another user with the same error in 2021, but no answer as to how to fix it. Here is the issue:
After installing the emulator, it wouldn't run. So I ran Terminal, found the app folder, then found aiStarter. I ran, from Terminal, ./aiStarter, and this is what it told me:
MacBook-Pro:commands-for-Appinventor tomdowad$ ./aiStarter
[621] Error loading Python lib '/var/folders/mx/27frfycj64538crxj2h3lvj40000gn/T/_MEIp7VIii/.Python': dlopen: dlopen(/var/folders/mx/27frfycj64538crxj2h3lvj40000gn/T/_MEIp7VIii/.Python, 10): Symbol not found: _getentropy
Referenced from: /var/folders/mx/27frfycj64538crxj2h3lvj40000gn/T/_MEIp7VIii/.Python (which was built for Mac OS X 10.12)
Expected in: /usr/lib/libSystem.B.dylib
in /var/folders/mx/27frfycj64538crxj2h3lvj40000gn/T/_MEIp7VIii/.Python
There should be some version numbers too - the latest version of El Capitan is 10.11.6. It's old, 2015, and Apple no longer support it (same as Microsoft no longer supporting my Windows 7, but App Inventor does).
I think it may still possible to upgrade your OS, should that be necessary, let's see what Evan or Susan think.
There is a build of the emulator package floating around that is built for 10.10 and later. I'm not sure whether that is the version I have currently installed but I will see if I can locate the installer.