Hi rhino-led, welcome to Thunkable! 
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I’m not sure why you’re seeing this option–that should be for internal use only. Please use the option below this one that just says “Download iOS”
However, to be able to download your app to iOS, you do need to have an Apple Developer account and you would have to turn on Developer Mode on your device.
You can find more instructions about how to download your app for iOS in our Docs: Download Your App on iOS | Thunkable Docs