Thanks for taking a look. It was Section 2, Step 4, having typed in the 'struct LandmarkRow_Previews ...' code that the problem occurred. I have re-opened the project and selected LandmarkRow view. The preview canvas states 'Preview paused'. Clicking the refresh arrow again throws the error "Landmarks quit unexpectedly". Upon clicking 'Reopen', get "You can't use this version of the application "Landmarks" with this version of macOS". I think I'd better try the tutorial again from scratch.