Connect to an AWS Instance Via SSH

Create and Download a .PEM file for the instance

Follow the instructions by Amazon

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connect-linux-inst-ssh.html

Make sure you are using the right username

For Amazon Linux 2 or the Amazon Linux AMI, the username is ec2-user. 
For a CentOS AMI, the user name is centos.
For a Debian AMI, the user name is admin.
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 a Ubuntu AMI, the user name is ubuntu.
For an Oracle AMI, the user name is ec2-user.
For a Bitnami AMI, the user name is bitnami.
Otherwise, if ec2-user and root don't work, check with the AMI provider.

Try to connect

ssh -i your_key.pem username@ip address