We're developing a new version of the app written entirely in SwiftUI, while the previous version was written in Objective-C. However, my company needs to ensure that the app can run on iOS 12. Is there any way to make SwiftUI compatible with iOS 12?
SwiftUI was introduced in iOS 13/watchOS 6/macOS Catalina. Your minOS must be 13.0/6.0/10.15 or higher to use SwiftUI.
If it is truly required to make your app available on iOS 12 as well, you could offer the last version submitted to the App Store prior to bumping your minOS to >= 13.0 to customers running iOS 12, but that would then be the last update available to them.
Some developers have found it feasible to shunt people down different code paths at app launch; depending on your app's architecture and complexity, this might be an option for you.