View the original community article here
Last tested: Nov 5, 2018
This error is indicative of this Looker trying and failing to start using the new GCM encryption. This means that a self-hosted or Looker hosted instance is trying to use AWS Key Management Service (KMS) for storing the encryption keys but the alias provided in the LKR_AWS_CMK environment variable is not known to KMS as being a valid key.
If Looker hosted, contact our Looker support team to ensure the alias is being sourced correctly and the key exists. If your instance is self-hosted, ensure the alias exists in KMS and that the Looker instance has permission to use it and is able to access KMS.