Please follow the steps mentioned below to get the connection string to connect to your cluster.
Click on Clusters
Click on CONNECT
Click on Connect with the Mongo Shell
Click on I have the Mongo Shell installed and then make sure you have selected 3.6 or later.
Finally click on Copy at the bottom to copy the connection string.
Paste the connection string in the terminal and the prompt will ask for password. Enter the password that you have set for the given user in your cluster.
For every account you can have just one free tier M0 cluster and that’s why you are not able to create another free tier cluster.
You do not need to create another cluster as you are already having one.
I would request you to go through this lab Lecture: Lab 2.0: Create an Atlas Sandbox Cluster (Ungraded) again and make sure you have not missed any steps.
Make sure you have updated your IP Whitelist (0.0.0.0/0) so that your app can talk to the cluster from all the locations.