Error creating DB Instance InvalidParameterCombination Cannot find version 5 6 10a for aurora-mysql

Question

any ideas why I got this error Error creating DB Instance: InvalidParameterCombination: Cannot find version 5.6.10a for aurora-mysql when executing the plan? The version 5.6.10a I got it from the aws mgt console. I tried aurora 5.7 and still failed.

resource "aws_db_instance" "test" {
    identifier                = "test"
    allocated_storage         = 1
    storage_type              = "aurora"
    engine                    = "aurora-mysql"
    engine_version            = "5.6.10a"
    instance_class            = "db.t2.small"
    name                      = "testdb"
    username                  = "testadmin"
    password                  = "xxxxx"
    port                      = 3306
    publicly_accessible       = false
    availability_zone         = "us-east-2a"
    security_group_names      = []
    vpc_security_group_ids    = ["sg-xxxxx"]
    db_subnet_group_name      = "default-vpc-xxxxx"
    parameter_group_name      = "default.aurora5.6"
    multi_az                  = false
    backup_retention_period   = 1
    #backup_window             = "08:48-09:18"
    #maintenance_window        = "sun:08:56-sun:09:26"
    final_snapshot_identifier = "test-final"
}