I tried both Xcode 11.5.6 & 12 beta 5 with the same result:

The vary last line of this lesson

.self.presentationMode.wrappedValue.dismiss()

produces a a compile error - "Value of tupletype '()' has no member 'presentationMode'

Any ideas please. AddBookView code below:

import SwiftUI

struct AddBookView: View { // MARK: - PROPERTIES

@Environment(\.managedObjectContext) var moc
@Environment(\.presentationMode) var presentationMode

@State private var title = ""
@State private var author = ""
@State private var rating = 3
@State private var genre = ""
@State private var review = ""
let genres = ["Fantasy", "Horror", "Kids", "Mystery", "Poetry", "Romance", "Thriller", "Spy"]

// MARK: - BODY

var body: some View {
    NavigationView {
        Form {

            // MARK: _ SECTION 1
            Section {
                TextField("Name of Book", text: $title)
                TextField("Author's name", text: $author)

                Picker("Genre", selection: $genre) {
                    ForEach(genres, id: \.self) {
                        Text($0)
                    }
                }//: PICKER
            }//: SECTION 1
            // MARK: _ SECTION 2
            Section {
                Picker("Rating", selection: $rating) {
                    ForEach(0..<6) {
                        Text("\($0)")
                    }
                }
                TextField("Write a Review", text: $review)

            }//: SECTION 2
            // MARK: _ SECTION 3

            Section {
                Button("Save") {
                let newBook = Book(context: self.moc)
                newBook.title = self.title
                newBook.author = self.author
                newBook.rating = Int16(self.rating)
                newBook.genre = self.genre
                newBook.review = self.review

                try? self.moc.save()

                    .self.presentationMode.wrappedValue.dismiss()
                }
            }//: SECTION 3
        } //: FORM
        .navigationBarTitle("Add Book")
    }//: NAVIGATION
}//: VIEW

}//: STRUCT

// MARK: - PREVIEW

struct AddBookView_Previews: PreviewProvider { static var previews: some View { AddBookView() } }

Have you removed the first . from this line? I think this is a typo and should be without the .

Thank you @Hatsushira! One period/fullstop can cost a day's searching. I must have looked at it 50 times.