Hello everyone!

struct MissionShotView: View {
    let astronauts: [String: Astronauts] = Bundle.main.decode("astronauts.json")
    let missions: [Missions] = Bundle.main.decode("missions.json")

    @State private var showingList = false

    var body: some View {
        NavigationView {
            VStack {
                if showingList {
                    ListLayout(astronauts: astronauts, missions: missions) // Cannot convert value of type '[Missions]' to expected argument type 'Missions'
                } else {
                    GridLayout(astronauts: astronauts, missions: missions) // Cannot convert value of type '[Missions]' to expected argument type 'Missions'
                }
            }

GRID LAYOUT:

struct GridLayout: View {
    let astronauts: Astronauts
    let missions: Missions

    let columns = [
        GridItem(.adaptive(minimum: 150))
    ]

    var body: some View {
        ScrollView {
            LazyVGrid(columns: columns) {
                ForEach(missions, id: \.id) { mission in // Key path value type '_' cannot be converted to contextual type '_'
                                              // Generic struct 'ForEach' requires that 'Missions' conform to 'RandomAccessCollection'
                    NavigationLink {
                        MissionView(mission: mission)
                    } label: {

Same problem in ListLayout

What am I doing wrong?

@TooMaa has a coding problem:

Error: Cannot convert value of type '[Missions]' to expected argument type 'Missions'

When I coach programmers, I steer them away from short repetitive class and variable names. Instead, I encourage them to use longer, descriptive names.

Why? Yours is a great example where short names may have tripped you up. You're thinking you have a problem with understanding Swift, or SwiftUI. But I think the problem is with your variables leading you astray.

What is the difference between mission, missions, Missions, and [Missions] ?
I think you are confusing yourself!

You have a json file that contains a large collection of mission objects.
You call that collection missions. But you also call your mission class Missions

let missions: [Missions] = Bundle.main.decode("missions.json")  // Ooooof! Much duplicate. So confusion.

How would more descriptive names help you?

I might recommend you call your class: SingleMission
This contains all the data, and functions that support one, unique, single mission.

Then when you decode the JSON file, you have a collection of SingleMission objects, like this: [SingleMission] Consider giving this variable a descriptive name such as apolloMissions.

// this tells you that apolloMissions is a collection (array) of SingleMission objects.
let apolloMissions: [SingleMission] = Bundle.main.decode("missions.json")

Now, what exactly do you want to send to your GridLayoutView ? A single mission? or a collection of single missions?

Much Confusion

In your definition, you declare your GridLayout view to have a variable named missions. You define this variable to contain just one Missions object. Indeed, you are initializing the GridLayout with a variable also named missions. But that missions object is a collection of Missions objects. Single object versus collection of objects.

Yuck! Do you see how confusing this is?

// Your code is confusing you....
// all the missions
let missions: [Missions] = Bundle.main.decode("missions.json")

// ... snip .....
// You initialize a GridLayou like this...
// You send in data via the missions variable. missions suggests MANY missions.
        } else {
            GridLayout(astronauts: astronauts, missions: missions)  // <- Send in data via the missions variable
        }

struct GridLayout: View {
    let astronauts: Astronauts
    // Your view only expects ONE Missions (misnamed. Should be Mission (singular)
    let missions: Missions   // <-- This (plural) variable expects a single object. But the name suggest MULTIPLE objects.
    // ....snip.....
}

More Descriptive Names

If you revise your code, and rename your classes, structs, and variables, you'll have a better understanding.

//  Declare your SingleMission struct
struct SingleMission {
    // This contains data and functions to support one and only ONE mission.
}
// Clearly state that apolloMissions is a collection of many SingleMission objects
let apolloMissions = [SingleMission]  // <-- Redefine

// Declare GridLayout
struct GridLayoutView : View {
    // This grid will show a collection of SingleMission objects
    let missionsInThisGrid : [SingleMission] 
    // ... snip ....
}

// Initialize the GridLayoutView
// ... snip ....
  } else {
      // With descriptive names, it's hard to make a mistake when initializing a new GridLayoutView
      // Your GridLayoutView expects an array of SingleMission objects.
      // You are providing an array of SingleMission objects
      GridLayoutView(astronauts: astronauts, missionsInThisGrid: apolloMissions)  // <-- No mistakes in initializer!
  }

Please let us know how you solved your problem.