If you still get same error with --authenticationDatabase admin , than probably your username and password are incorrect.
Try adding a user db.createUser() , with appropriate role ( i gave write permission as well)
than run below command : (ignore -h if you are running on local)
I was having this same problem when trying to dump my database info from mLab.
It was happening because I had mongo 2.x locally and 3.x in mLab.
Upgrading my local mongo to the same major version as mLab allowed me to do the dump, thus solving the problem.
If you are using mLab then it could be the version in your local mongo is not match with mLab. By default, Ubuntu will install mongo v2.x and mLab is v3.x. You could check with this command:
mongo --version
Install new mongo version:
Remove your old local mongo (be careful, it may remove all your local database)
sudo apt remove mongo-clients mongodb
Import the public key used by the package management system.