If you run into problems, you can enter the full menu path with a hyphen and right arrow to divide each menu, for example, Library->Previews->Build Standard-Sized Previews. To type an ellipsis, use three periods without spaces.
In the Menu Title field, type the menu command, for example, Build Standard-Sized Previews. You must type the command exactly as it appears in the menu, including ellipses (such as Synchronize Folder…) and any other punctuation.If it doesn’t show up, scroll down to the bottom, select Other and navigate to Applications > Adobe Lightroom > Adobe Lightroom.app. Select Lightroom in the Application pop-up.Select App Shortcuts and click the + button.Open the Keyboard preferences and select the Shortcuts tab.Go to Applications > System Preferences.