Help in exercice tracker project of APIs microservices

when i opened the project in repl.it , 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 https://www.freecodecamp.org/learn/apis-and-microservices/apis-and-microservices-projects/exercise-tracker

Hey there,

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