From what it seems, you're using the wrong SSH key. You launched the Bitnami Open edX instance via the AWS Marketplace, right?
I recommend you to go to your EC2 Management Console, click on your instance, and check the "Key pair name" property under "Description". You should see the SSH key which was configured with your instance.
If it is not recognized or you don't have access to that key, unfortunately, you will not be able to access that instance, so you will need to create a new instance (and migrate any data to it, if you added any) if you want to have SSH access.