I have an MacBook with El Capitan installed on it. I cannot upgrade to sierra! Because of that, I cannot install XCode 8.3.
Now I need to test an App with iOS 10.3 but I cannot build it with XCode 8.2.1 for iOS 10.3. I have an pyhsical device with iOS 10.3 but XCode wont let me run it on that device.
What to do now?
Unfortunately, you're out of luck. As this answer specifies, iOS 10.3 requires Xcode 8.3 or greater, which in turn requires macOS Sierra.