The AEM Mobile Signing Tool shows the following in its iOS panel and iOS apps cannot be signed: "iOS apps cannot be signed because of the following issues: Xcode 8 or greater could not be found on your system. Xcode 8 is required for signing and submitting to the App Store.”
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:
sudo xcode-select -s /Applications/Xcode.app/Contents/Developer
Now launch the AEM Mobile Signing Tool.