I am getting the above error with this TabView code... Being the dummy that I am I don't know what it means...

 TabView(selection: $selectedTab) {
                    .tabItem {
                        Image(systemName: "plus")
                        showingAddDog = true
                        .foregroundColor(.blue)
                        }
                        .tag("One")

                    .tabItem {
                        Image(systemName: "list.dash.header.rectangle")
                        showingListDogView = true
                        .foregroundColor(.green)
                        }
                        .tag("Two")

            } // end of TabView

Solved !! After some looking thru Forums I found that . sheets don't work with TabView so I used a solution recommended by Nigel.

You getting that error because you have no views attached to .tabItem. Also you can not have showingAddDog = true and showingListDogView = true a .tabItem

var body: some View {
    TabView(selection: $selectedTab) {
        TabOneView() // <- Need a view
            .tabItem {
                Image(systemName: "plus")
                    .foregroundColor(.blue)
            }
        .tag("One")

        TabTwoView() // <- Need a view
            .tabItem {
                Image(systemName: "list.dash.header.rectangle")
                    .foregroundColor(.green)
            }
        .tag("Two")
    }
}