Could not cast value of type 'Swift.Array.

You don't show enough code (as usual 😊).

How is MapDataManager defined ? (one need to register to the Packt to see it). What is RestaurantItem. Does it have a property like mkAnnotation (I guess so)

But, in any case: manager.annotations is an array of MKAnnotation hence you cannot make it a single MKAnnotation

In addition, you pass annontations (with a typo for annotations) to addMap and never use it !

So following is a best guess :

May be you should have something like:

        func addMap(_ annotations: [RestaurantItem]) {    // 
            mapView.setRegion(manager.currentRegion(latDelta: 0.5, longDelta: 0.5), animated: true)
            for restaurant in annotations {
                mapView.addAnnotation(restaurant.mkAnnotation as! MKAnnotation)
           }
        }

So, please provide enough information to confirm.

Thank you for your reply! I did not add too many files until I got some feedback. I guess that you would like to see RestaurantItem.swift & MapDataManager.swift:

l//
//  RestaurantItem.swift
//  EatOut
//
//  Created by Tony Hudson on 12/07/2021.
//

import UIKit
import MapKit

class RestaurantItem: NSObject, MKAnnotation {
    var name: String?
    var cuisines: [String] = []
    var lat: Double?
    var long: Double?
    var address: String?
    var postalCode: String?
    var state: String?
    var imageURL: String?
    var restaurantID: Int?

    init (dict: [String: AnyObject]) {
        if let lat = dict["lat"] as? Double { self.lat = lat}
        if let long = dict["long"] as? Double { self.long = long}
        if let name = dict["name"] as? String { self.name = name}
        if let cuisines = dict["cuisines"] as? [String] { self.cuisines = cuisines}
        if let address = dict["address"] as? String { self.address = address}
        if let postalCode = dict["postalCode"] as? String { self.postalCode = postalCode}
        if let state = dict["state"] as? String { self.state = state}
        if let image = dict["imageURL"] as? String { self.imageURL = image}
        if let id = dict["id"] as? Int { self.restaurantID = id}
}

    var coordinate: CLLocationCoordinate2D {
        guard let lat = lat, let long = long else {
            return CLLocationCoordinate2D()
        }
    return CLLocationCoordinate2D(latitude: lat, longitude: long)
}
    var title: String? {
    return name
    }
    var subtitle: String? {
        if cuisines.isEmpty { return ""}
        else if cuisines.count == 1 { return cuisines.first}
        else { return cuisines.joined(separator: ", ")
        }
    }

}

//  MapDataManager.swift
//  EatOut
//
//  Created by Tony Hudson on 12/07/2021.
//

import Foundation
import MapKit

class MapDataManager: DataManager {
    fileprivate var items: [RestaurantItem] = []
    var annotations: [RestaurantItem] {
        return items
    }
    func fetch(completion: (_ annotations: [RestaurantItem]) -> ()){
    if items.count > 0 { items.removeAll() }
        for data in load(file: "MapLocations") {
    items.append(RestaurantItem(dict: data))
    }
    completion(items)
    }

    func currentRegion(latDelta: CLLocationDegrees, longDelta: CLLocationDegrees) -> MKCoordinateRegion {
        guard let item = items.first else {
            return MKCoordinateRegion()
        }
    let span = MKCoordinateSpan(latitudeDelta: latDelta, longitudeDelta: longDelta)
        return MKCoordinateRegion(center: item.coordinate, span: span)
    }
}

Could you explain what you said that annotations was not used?