The Mac OS has the "active developer directory" set to '/Library/Developer/CommandLineTools'. It is fine when using only the Xcode command line tools but the AEM Mobile Signing Tool needs the Xcode app. So it must be installed and, if necessary, it has "active developer directory" set to the app.
If you have not already done so, install Xcode from the Mac OS App Store application. Attempt to use the AEM Mobile Signing Tool.
If it still shows the error, do the following:
- In the finder go to /Applications/Utilities and launch the Terminal app.
- Type the following, then press Return:
sudo xcode-select -s /Applications/Xcode.app/Contents/Developer
Now launch the AEM Mobile Signing Tool.