JavaScript execution failed: Error: couldn't connect to server 127.0.0.1:27017 at src/mongo/shell/mongo.js:L112

If you are running Ubuntu, then there is an issue with folder ownership.

Run these commands:

    Stop mongodb service

                  sudo service mongodb stop

    Remove mongodb lock file

                   sudo rm /var/lib/mongodb/mongod.lock

    Change ownership from root to mongodb path

                 sudo chown -R mongodb:mongodb /var/lib/mongodb/

    Start mongodb service

                 sudo service mongodb start

    Test mongo app

                  mongo

Then you will be able to execute successfully.

   Refer:dba.stackexchange