Hello
You get this message from Apple when your app is configured to support versions of iOS on which some of the Swift APIs your app uses did not exist (or it can also be the Swift language version). In this case, the whole Swift runtime from the iOS SDK you use should be embedded in your app's IPA when it is created - this usually causes it to be much heavier.
The solutions are, from the simplest to the most complex:
1. To remove support for devices and iOS versions that are too old, on which the Swift language versions or the set of Swift APIs your app uses were not supported,
2. To rewrite your code so that it only uses a version of Swift *and* a set of Swift APIs that are guaranteed to be supported by the lowest iOS version your app claims to support,
3. To embed the Swift runtime that's in your iOS SDK in your app's IPA file. This is not done automatically by the builder (the drawbacks are sensible). This needs to be done manually. I can provide guidance with that if necessary.
*edit* about #2, it might not necessarily be your code: the culprit can also be the code of a framework, library or 3rd party asset your app uses.