Help in exercice tracker project of APIs microservices

when i opened the project in , i tried to run it but an error occurred and the problem is the
mongoose.connect(process.env.MLAB_URI || ‘mongodb://localhost/exercise-track’ )
when i deleted it no error occurred
so how can i do the project while the connection with mongoose made an error below you find the url of the project

Hey there,

what does the error exactly say?
Do you have this environment variable MLAB_URI with a valid url to your database?