The controller

console output

Compiled web/channels/room_channel.ex
[info] POST /api/v1/like_posts
[debug] Processing by MusicTester.LikePostController.create/2
  Parameters: %{"like_post" => "{ post_id: 15, user_id: 1}"}
  Pipelines: [:api]
"{ post_id: 15, user_id: 1}"

[info] Sent 500 in 7ms
[error] #PID<0.898.0> running MusicTester.Endpoint terminated
Server: localhost:4000 (http)
Request: POST /api/v1/like_posts
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in Ecto.Changeset.cast/5
        (ecto) lib/ecto/changeset.ex:361: Ecto.Changeset.cast(%MusicTester.LikePost{__meta__: #Ecto.Schema.Metadata<:built>, id: nil, inserted_at: nil, post: #Ecto.Association.NotLoaded<association :post is not loaded>, post_id: nil, updated_at: nil, user: #Ecto.Association.NotLoaded<association :user is not loaded>, user_id: nil}, %{}, "{ post_id: 15, user_id: 1}", [], [])
        (music_tester) web/controllers/like_post_controller.ex:14: MusicTester.LikePostController.create/2
        (music_tester) web/controllers/like_post_controller.ex:1: MusicTester.LikePostController.action/2
        (music_tester) web/controllers/like_post_controller.ex:1: MusicTester.LikePostController.phoenix_controller_pipeline/2
        (music_tester) lib/phoenix/router.ex:261: MusicTester.Router.dispatch/2
        (music_tester) web/router.ex:1: MusicTester.Router.do_call/2
        (music_tester) lib/music_tester/endpoint.ex:1: MusicTester.Endpoint.phoenix_pipeline/1
        (music_tester) lib/plug/debugger.ex:93: MusicTester.Endpoint."call (overridable 3)"/2
        (music_tester) lib/phoenix/endpoint/render_errors.ex:34: MusicTester.Endpoint.call/2
        (plug) lib/plug/adapters/cowboy/handler.ex:15: Plug.Adapters.Cowboy.Handler.upgrade/4
        (cowboy) src/cowboy_protocol.erl:442: :cowboy_protocol.execute/4

like_post.ex

defmodule MusicTester.LikePost do
  use MusicTester.Web, :model

  schema "like_posts" do
    belongs_to :user, MusicTester.User
    belongs_to :post, MusicTester.Post

    timestamps
  end

  @required_fields ~w()
  @optional_fields ~w()

  @doc """
  Creates a changeset based on the `model` and `params`.

  If no params are provided, an invalid changeset is returned
  with no validation performed.
  """
  def changeset(model, params \\ :empty) do
    Apex.ap params
    model
    |> cast(params, @required_fields, @optional_fields)
  end
end

like_post_controller.ex

def create(conn, %{"like_post" => like_post_params}) do
   changeset = LikePost.changeset(%LikePost{}, like_post_params)
   
   case Repo.insert(changeset) do
     {:ok, like_post} ->
       conn
       |> put_status(:created)
       |> put_resp_header("location", like_post_path(conn, :show, like_post))
       |> render("show.json", like_post: like_post)
     {:error, changeset} ->
       conn
       |> put_status(:unprocessable_entity)
       |> render(MusicTester.ChangesetView, "error.json", changeset: changeset)
   end

post.html.slim

= link gettext("Like"), to: "#", class: "like-post", id: @post.id, value: "{ post_id: #{@post.id}, user_id: #{ @current_user.id}}"

router.ex

defmodule MusicTester.Router do
  use MusicTester.Web, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
    plug MusicTester.Locale
    plug MusicTester.Auth, repo: MusicTester.Repo
    plug MusicTester.Navbar
    plug MusicTester.Breadcrumbs
  end

  pipeline :api do
    plug :accepts, ["json"]
    
  end
  
  scope "/api/v1" do
     pipe_through :api 
     resources "/like_posts", MusicTester.LikePostController
  end

  scope "/", MusicTester do
    pipe_through :browser # Use the default browser stack

    get "/", PageController, :index
    resources "/users", UserController, except: [:index]
    resources "/forgot_password", ForgotPasswordController, only: [:index, :create]
    resources "/reset_password", ResetPasswordController, only: [:show, :update]
    resources "/sessions", SessionController, only: [:new, :create, :delete]

    resources "/posts", PostController, only: [] do
      resources "/comments", CommentController, exclude: [:show, :index]
    end
    
    resources "/topics", TopicController, exclude: [:show] do
      resources "/posts", PostController 
    end
    
    resources "/comments", CommentController, only: [] do
      resources "/replies", ReplyController, exclude: [:index]
    end

    resources "/works", WorkController, exclude: [:show]  do
      resources "/recordings", RecordingController
    end
    
    resources "/like_posts", LikePostController#, except: [:new, :edit]
    
  end

  # Other scopes may use custom stacks.
  scope "/admin", MusicTester do
    pipe_through [:browser, :authenticate_admin]
    resources "/", AdminController, only: [:index]
    resources "/users", UserController, only: [:index]
    #pipe_through :api
    resources "/banners", BannerController, except: [:show]
    resources "/categories", CategoryController, except: [:show]
    resources "/cards", CardController, except: [:show]
  end
  
  
end

shared.js

$(".like-post").click(function() {
  let csrf = document.querySelector("meta[name=csrf]").content;
  let params = $(this).attr('value')

  $.ajax({
      url: "/like_posts",
      type: "POST",
      params: {like_post: params},
      data: {
        like_post: params
      },
      headers: {
          "X-CSRF-TOKEN": csrf 
      },
      dataType: "json",
      success: function (data) {
        console.log(data);
      },
      error: function (data) {
        console.log(data);
      }
  });
});