The great programming answer:
It depends– Every programmer ever
It depends on what your goals are. If you are wanting to get a job in the next year or two, learn UIKit. If you are just playing around with iOS development learn SwiftUI.
This is not to say someone who wants an iOS job shouldn’t learn SwiftUI, you can of course learn both congruently. However there’s a 98% chance the projects you work on will be UIKit.
SwiftUI was released in 2019. It will take many years for it to overthrow UIKit apps. Think about it, Swift was released in 2014. It’s now 2020. There are MANY apps still written with Objective-C, and it’s been 6 years. Adoptions of new technologies like these don’t happen overnight.