Hi,
This issue could be related to any older plugins that require a specific version of SWIFT_VERSION. It would be helpful to evaluate your plugins for any such.
If you are using Cordova, you can also set preference In your config.xml .
<platform name="ios">
<preference name="UseSwiftLanguageVersion" value="5" />
If you are facing any further issues, I would encourage you to contact the Appflow support for further assistance.
Thanks