If “Use second display” is disabled in the Preview window, a message at the bottom of the window indicates what you need to do to enable dual-screen use. The requirements are:

  1. No other app can be in fullscreen.
  2. The second display needs to run at a resolution smaller or equal to that of the main display.
  3. “Mirror Displays” (which is unrelated to use of a mirror enclosure) must be turned off in the Displays System Preferences.

If the required change is simple, a one-click button is provided similar to the following:

Quickfix button to enable use second display

The “Change Resolution” button chooses what Presentation Prompter thinks is the best-compatible resolution. If there are additional compatible resolutions, the button has a pulldown menu beside it for your choosing.

Change resolution button showing menu items

If a “System Preferences” button is provided or you want to make screen adjustments manually, you need to make display changes similar to the following:

Displays System Preferences with dual-screen requirements

On some versions of macOS, you may need to hold the Option key and click “Scaled” to show additional display resolutions and find something compatible:

Displays System Preferences showing additional resolutions