Firstly :

Do not include Reality Composer's .rcproject files in your archive for distribution. .rcproject bundles contain the code with iOS 13.0+ classes, structs and enums. Instead, supply your project with USDZ files.

Secondly :

To allow iOS 13+ users to use RealityKit features, but still allow non-AR users to run this app starting from iOS 10.0, use the following code:

import UIKit

#if canImport(RealityKit)
import RealityKit
import Combine

@available(iOS 13.0, *)
class ViewController: UIViewController {
    
    var arView = ARView(frame: .zero)
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        arView.frame = self.view.frame
        self.view.addSubview(arView)
                
        let entity = ModelEntity(mesh: .generateBox(size: 0.1))
        let anchor = AnchorEntity(world: [0,0,-2])
        anchor.addChild(entity)
        arView.scene.anchors.append(anchor)
    }
}

#else
import SceneKit

#endif

Deployment target is iOS 10.0:

Thirdly :

When publishing to the AppStore (in case we have a deployment target lower than iOS 13.0), we must make the import of this framework weakly linked in the build settings (that's because RealityKit is deeply integrated in iOS and Xcode).

So, go to Build Settings –> Linking -> Other linker Flags.

Double-click it, press +, and paste the following command:

-weak_framework RealityKit -weak_framework Combine

P.S. In Xcode 13.3, there's a project setting that also could help

OTHER_LDFLAGS = -weak_framework RealityFoundation

Fourthly :

So, go to Build Settings –> Framework Search Paths.

Then type there the following command:

$(SRCROOT)

it must be recursive.

Fifthly

The archives window: