Hi,
Click the "Connect" button from within your MongoDB Atlas cluster section, there you should find an option named "Connect with MongoDB Compass" where you're able to copy the connection URI string of the cluster.
When you open MongoDB Compass, it would detect whether there is a MongoDB connection URI string on your clipboard. If found, MongoDB Compass will ask and try to auto-populate the connection form. See MongoDB Atlas: Compass Connection for more information.
Alternatively, you can also connect manually to MongoDB Compass. Please make sure that you're specifying the correct value for the SSL field.
If you're still having an issue connecting to MongoDB Atlas, try connecting using mongo shell first to debug. See also MongoDB Compass Contact.
Regards,
Gaurav