RDS encryption has not been enabled at a DB Instance level.
Explanation
Encryption should be enabled for an RDS Database instances.
When enabling encryption by setting the kms_key_id.
Insecure Example
The following example will fail the AWS052 check.
resource "aws_db_instance" "my-db-instance" {
}
Secure Example
The following example will pass the AWS052 check.
resource "aws_db_instance" "my-db-instance" {
storage_encrypted = true
}