I'm not sure what your desired outcome is, because the "error" is pretty user-friendly and clear to me. If you want to make your app compatible with earlier OS versions then Wilfried is right, though it has a different name for Phonegap. You can check the config.xml (which is part of the PhoneGap build package) to see which PhoneGap version you're using:
<preference name="phonegap-version" value="cli-6.3.0" />
By lowering the value your app becomes available for older OS versions. For an overview of available versions I suggest checking out this overview
It does appear to be an issue with android-maxSdkVersion not being set correctly in the AndroidManifest.xml file. Thanks for your responses.