Post a screenshot of what you are trying that shows the issues you are getting.
Without an exact error message we cannot really help.
Just in case, and are placeholders and must be replaced with your real admin user password and a real database name. It is the safest to use test as the database name if you are unsure.
If you look at the documentation of mongodump at https://docs.mongodb.com/database-tools/mongodump/ you will notice in the examples that they have an equal sign between –uri and the actual URI.