Well, MERN will not be MERN without Mongo. 😊
There is no nifty command for it in mern-cli, however to exclude Mongo from it you need to identify the parts that involve mongo. You will need to change models, controllers, routes and change server.js to whatever database that you need to use or simply remove them if you are using some API.
@adrianhartanto More info about the error will be helpful. Are other pages loading? Make sure you have setup correct routes.