Creating Keycloak Realm via Ansible
As @JSapkota mentioned the URL was wrong. It must be https://keycloak-server/auth/admin/realms.

- name: "Create Realm for service Keycloak"
  uri:
    url: "https://keycloak-server/auth/admin/realms"
    method: POST
    src: "realm.json"
    remote_src: "no"
    status_code:
     - 201
    headers:
      Content-type: "application/json"
      Accept: "application/json"
      Authorization: "Bearer {{ keycloak_token.json.access_token }}"
  register: keycloak_realm_create


Take care about the return status code. It is "201 created" and the uri module must know, that this is fine.

This works one time. Because in the next run the REST API responses with 409 the uri module fails. 
To be idempotent, you need to check first, if the realm exists. This can be done via GET /auth/admin/realms/{{ keycloak_realm_name }} and check the return code, if it is 404 or 200. If it is 404, then you make a "POST" otherwise a "PUT". So, this a complete example:

- name: "Set facts"
  set_fact:
    keycloak_admin_user: "admin"
    keycloak_admin_pass: "password"
    keycloak_base_url: "https://keycloak.server"
    keycloak_realm_name: "myrealm"
    keycloak_realm_data_file: "realm.json"

- name: "Create Token for service Keycloak"
  uri:
    url: "{{ keycloak_base_url }}/auth/realms/master/protocol/openid-connect/token"
    method: POST
    body_format: form-urlencoded
    body:
      username: "{{ keycloak_admin_user }}"
      password: "{{ keycloak_admin_pass }}"
      grant_type: "password"
      client_id: "admin-cli"
  register: keycloak_token

- name: "Find out, if Realm {{ keycloak_realm_name }} for service Keycloak exists"
  uri:
    url: "{{ keycloak_base_url }}/auth/admin/realms/{{ keycloak_realm_name }}"
    method: GET
    status_code:
     - 200
     - 404
    headers:
      Accept: "application/json"
      Authorization: "Bearer {{ keycloak_token.json.access_token }}"
  register: keycloak_realm_exists

- name: "Create Realm {{ keycloak_realm_name }} for service Keycloak"
  uri:
    url: "{{ keycloak_base_url }}/auth/admin/realms"
    method: POST
    src: "{{ keycloak_realm_data_file }}"
    remote_src: "no"
    status_code:
     - 201
    headers:
      Content-type: "application/json"
      Accept: "application/json"
      Authorization: "Bearer {{ keycloak_token.json.access_token }}"
  register: keycloak_realm_create
  when: "keycloak_realm_exists.status == 404"

- name: "Update Realm {{ keycloak_realm_name }} for service Keycloak"
  uri:
    url: "{{ keycloak_base_url }}/auth/admin/realms/{{ keycloak_realm_name }}"
    method: PUT
    src: "{{ keycloak_realm_data_file }}"
    remote_src: "no"
    status_code:
     - 204
    headers:
      Content-type: "application/json"
      Accept: "application/json"
      Authorization: "Bearer {{ keycloak_token.json.access_token }}"
  register: keycloak_realm_create
  when: "keycloak_realm_exists.status == 200"


Also I had the problem, that after every request the Keycloak server responses with a 403 when I don't refresh the token before the next call to Keycloak. But I think, this depends on Realm token settings of the admin realm.