Why am I getting a "missing ) after argument list" when connecting to Heroku?

Question

my MONGO_URI

mongoose.connect(mongodb+srv:${process.env.DB_USERNAME}:${process.env.DB_PASSWORD}@rybnetwork.sn5ae.mongodb.net/rybnetwork, {useNewUrlParser: true,
  useUnifiedTopology: true
});

I don't have a MONGO_URI in my dotenv. I only have DB_USERNAME and DB_PASSWORD and PORT

my error message:

mongoose.connect(mongodb+srv:${process.env.DB_USERNAME}:${process.env.DB_PASSWORD}@rybnetwork.sn5ae.mongodb.net/rybnetwork, {useNewUrlParser: true,
                         ^^^

or should I ignore correcting this since with Heroku I'd have to use MTLS PostgreSQL?

Just prior to the above error message, I received:

(node:174556) UnhandledPromiseRejectionWarning: MongoParseError: Invalid connection string at parseConnectionString