Just a comment.
In the Session Manager lab session, all works fine because the Amazon Linux that was used already comes with the SSM Agent pre-installed.
For other instances, it is necessary to manually install the SSM Agent in order for Session Manager to start working.
sudo yum install -y https://s3.amazonaws.com/ec2-downloads-windows/SSMAgent/latest/linux_amd64/amazon-ssm-agent.rpm [FOR INTEL]
sudo systemctl status amazon-ssm-agent
sudo systemctl enable amazon-ssm-agent (optional)
sudo systemctl start amazon-ssm-agent (optional)