AWS EC2 Connection closed by when trying ssh into instance

Often times, you should use the default user name for the AMI that you used to launch your instance:

• For Amazon Linux 2 or the Amazon Linux AMI, the user name is
  ec2-user.

• For a CentOS AMI, the user name is centos.

• For a Debian AMI, the user name is admin or root.

• For a Fedora AMI, the user name is ec2-user or fedora.

• For a RHEL AMI, the user name is ec2-user or root.

• For a SUSE AMI, the user name is ec2-user or root.

• For an Ubuntu AMI, the user name is ubuntu.

  Otherwise, if ec2-user and root don't work, check with the AMI provider.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connection-prereqs.html#connection-prereqs-get-info-about-instance

My problem was that the username was incorrect. On ubuntu instances it should be "ubuntu" and on amazon instances it should be "ec2-user".

Please check your username. if you are using ubuntu instance try using "ubuntu" in username instead of "ec2-user", and vice-versa