Failed to connect to your instance Error establishing SSH connection to your instance. Try again later.

This error message is common when trying to connect to an EC2 instance using SSH or EC2 Instance Connect. There are several potential causes and solutions to address this issue:

1. Security Group Configuration: Ensure that your instance's security group allows inbound SSH traffic (port 22) from your IP address or the EC2 Instance Connect IP range for your specific region.

2. Network Settings: Verify that your instance is in a public subnet with an internet gateway attached, allowing for outbound traffic necessary for Instance Connect to function.

3. Instance State: Confirm that your EC2 instance is in the "running" state and has passed all status checks.

4. SSH Service: Check if the SSH service is running on the instance. You may need to restart it if it's not running.

5. Firewall Rules: Verify that the instance's firewall allows incoming connections on port 22.

6. Key Pair: Ensure you're using the correct private key file when attempting to connect via SSH.

7. Default User: Double-check that you're using the correct default user for your OS (e.g., "ec2-user" for Amazon Linux, "ubuntu" for Ubuntu, or "centos" for CentOS).

8. EC2 Instance Connect Prerequisites: If using EC2 Instance Connect, make sure your instance meets all the prerequisites, including having the necessary software installed.

To resolve the issue:

1. Update your security group's inbound rules to allow SSH traffic (TCP port 22) from your IP address and the EC2 Instance Connect IP range for your region.

2. Double-check your VPC and subnet configurations to ensure proper internet connectivity.

3. Try rebooting your instance if the problem persists.

4. Use verbose mode (ssh -v) when connecting to get more detailed error information.

5. Consider using alternative connection methods like AWS Systems Manager Session Manager if SSH continues to fail.

If you've verified all these settings and still can't connect, it may be worth reaching out to AWS support for more in-depth troubleshooting assistance.
Sources
Failed to connect to your instance | AWS re:Post
EC2 instance failed to connet | AWS re:Post
Failed to connect to your instance | AWS re:Post
Not able to connet CentOS instance | AWS re:Post