If Session Manager is not working, first verify that the Session Manager is configured. To do this:
Go to Amazon Systems Manager
If prompted to set it up, click the button to perform the initial setup and wait until setup completes (about 10 minutes).
From this point forward, new EC2 instances will have the credentials loaded by default into the instance to connect.
For existing instances created before the Systems Manager was initialized, credentials will need to be added. Very good step by step instructions are provided by Amazon. Search for ""Verify or add instance permissions for Session Manager"