Bad Request -- 400

I have been trying to signup using /dbconnections/signup, below is the data request body:
{
“client_id”: “xxx”,
“connection”: “xxx”,
“email”: “test@gmail.com”,
“password”: “Testing1”,
“user_metadata”: {
“givenName”: “test”,
“familyName”: “test”,
“marketingOptIn”: “false”,
“marketingOptInBusiness”: “false”,
“market”: “uk-en”,
“registerClientId”: “xxx”
}
}

Rules has been turn off
return:
{
“fromSandbox”: true,
“statusCode”: 400
}