{500 service unavailable (with message)} error when trying to register a user with API I want to register a user on drupal site through REST, 
I read this article, and implemented the instructtins exactly the same using Postman on drupal 8.3.

specially when I send this specification :

POST: http://example.com/user/register
Content-type: application/json
Data: { "name": [
  {
    "value": "test"
  }
],
"mail": [
  {
    "value": "aa@bb.com"
  }
],
"pass": [
  {
    "value": "test"
  }
]  }


Response is: 500 service unavailable (with message)

What should I do to solve the problem?
Just a stab in the dark here.

It seems that you have extra square brackets. I think this is why your error message refers to field 0 -- you're sending it an array. Try it exactly as the article displays, with no square brackets?

For example: 
    "pass": {
        "value": "test"
      }

Good luck.

Update

To enable setting a password, you need to change a setting which is default in the install.

Go to admin/config/people/accounts (Configuration > People > Account Settings) and uncheck  "Require email verification when a visitor creates an account":



Granted you are allowing POST to Anonymous users as described in the article above, you should be good to go and get a response such as the following in PostMan:

{
  "uid": [
    {
      "value": "5"
    }
  ],
  "uuid": [
    {
      "value": "70738718-d4e9-455e-a1f6-cf95df613507"
    }
  ],
  "langcode": [
    {
      "value": "en"
    }
  ],
  "name": [
    {
      "value": "ac@bb.com"
    }
  ],
  "created": [
    {
      "value": 1487319576
    }
  ],
  "changed": [
    {
      "value": 1487319576
    }
  ],
  "default_langcode": [
    {
      "value": true
    }
  ],
  "user_picture": []
}


Hope this helps!!