You probably have an older “Classic” App, which are being deprecated and can no longer be copied.
To be able to do either (duplicate or create template), you would first need to convert your Classic App to a new App. You can get a head start on this by using the “New app from data” option. This will create a new App with all of your data and computed columns intact, although you would then need to rebuild most of the user interface. See below for more on this: