You should better keep your code properly indented.
Cmd-A Ctrl-I
That will reveal some simple mistakes.
Value of type 'String' has no member 'frame' : line 174
Cannot infer contextual base in reference to member 'all': line 176
Your line 170...180 will look like this when extra whitespaces and newlines removed:
Text(isDecrementOn ? "-10" : "+10".frame(width: 50).padding(.all) )
You write .frame(width: 50) immediately after a String literal "+10".
You may want to add .frame to Text, not a String.
Closure containing a declaration cannot be used with function builder 'ViewBuilder' : line 186
The line struct ContentView_Previews: PreviewProvider { needs to be placed outside of the definition of struct ContentView.
Meaning you need to write it after the closing brace } matching the opening brace struct ContentView: View {.
You are writing struct ContentView_Previews: PreviewProvider { just after the closing brace for HStack(alignment: .center, spacing: 10) {.
You need to be careful enough about matching parentheses.