MERN Stack project database part does not seem to work

I am struggling to find out the way to run MERN projects cloned from GitHub. The part where I get stuck is the database. I am able to run the front-end and the back-end also says connection to database is successful ( which essentially means my connection string is working…) , but when i run the application and try to perform any Insert, Update, Delete operation, it doesn’t seem to work. It will be really helpful if you explain to me the way to run these cloned GitHub projects locally ( in correct way).