One of the most common problems that will trigger the Error Code 0x204 is an instance in which the Remote Desktop Protocol is not enabled on the Windows machine. In this case, you can fix the issue by enabling it manually via the System Properties screen and by allowing Network Level connections.
I would suggest you to check the following
As AWS-User-5291433 mentioned, check if Session manager works
Check the security group allows traffic from your computer to port 3389 (RDP).
Check if the instance health checks have passed
Check that your instance has a public IPv4 address.
Check if the instance is in a public subnet and check the route table for the subnet
Check the network access control list (ACL) for the subnet.
As a last step, check if Remote Desktop Protocol is enabled but this can only be done once you access through session manager
If none of these helped, please provide the console output for your instance using the Amazon EC2 console, select the instance, and then choose Actions, Monitor and troubleshoot, Get system log
To troubleshoot using the AWSSupport-TroubleshootRDP document
Log in to the Systems Manager Console. Verify that you are in the same Region as the impaired instance. Open the AWSSupport-TroubleshootRDP document. For Execution Mode, choose Simple execution. For Input parameters, InstanceId, enable Show interactive instance picker. Choose your Amazon EC2 instance. Review the examples, then choose Execute. To monitor the execution progress, for Execution status, wait for the status to change from Pending to Success. Expand Outputs to view the results. To view the output of individual steps, in Executed Steps, choose an item from Step ID.
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