I can not save values using POST method - laravel vue

I'm trying to save the data I send from the Event view. in the storeEvent method of the EventController driver but it gives me error 422 and I can not find the problem so far.

The Event model has a many-to-many relationship with the Categories model, and Event also has a many-to-many relationship with the Coins model, which is why I occupy vue-multiselect since the user can select several categories or several coins for the same event

Event.vue

<template>
  <form v-on:submit.prevent="createdEvent" class="form-horizontal">
   <div class="form-group row">
     <label>Titulo</label>
     <input type="text" name="title" maxlength="25" v-model="title">
   </div>
   <div class="form-group row">
     <label>*Cryptodivisas</label>
     <multiselect v-model="coinvalue" :options="coins"
                :multiple="true" label="name" 
                track-by="id" placeholder="Seleccione">
     </multiselect>
   </div>
   <div class="form-group row">
     <label>*Categoría</label>
     <multiselect v-model="categoryvalue" :options="categories"
                :multiple="true" label="name" 
                track-by="id" placeholder="Seleccione">
     </multiselect>
   </div> 
   <div class="col-sm-12">
     <button class="btn" type="submit">Crear evento</button>
   </div>
 </form>

<script>
    import Multiselect from 'vue-multiselect';
    export default {  
      components: { 
        Multiselect,
      },
      props: ['auth'],     
      data () {
        return {
            user:           {},
            title:          '',
            coins:          [],
            coinvalue:      [],
            categories:     [],
            categoryvalue:  [], 
       }
      },
      created() {
        this.getCoins();
        this.getCategories();
      },
      mounted() {
        this.user = JSON.parse(this.auth);
      },
      methods: {
        getCoins(){
            let urlCoin = '/dashboard/coins';
            axios.get(urlCoin)
                .then((response) => {
                    this.coins = response.data;
                })
                .catch((err) => {

                })
        },
        getCategories(){
            let urlCategories = '/dashboard/categories';
            axios.get(urlCategories)
                .then((response) => {
                    this.categories = response.data;
                })
                .catch((err) => {

                })
        },
        createdEvent(){
            let urlEvent = '/dashboard/newEvent';
            const eventData = {
                'id'            : this.user.id,
                'title'         : this.title,
                'coin'          : this.coinvalue,
                'category'      : this.categoryvalue,
            }
            console.log(eventData);
            axios.post(urlEvent, eventData)
                .then((response) => {
                    console.log(ok);
                })
                .catch((err) => {
                    console.log(err.response.data);
                })
        }
 </script>

storeEvent (EventController)

 public function storeEvent(Request $request)
 {
      $this->validate($request, [
                'title'    => 'required|max:25',
                'coin'     => 'required',
                'category' => 'required',
        ]);

        $userAuth = Auth::user()->id;
        $userEvent = $request->id;
        if($userAuth === $userEvent){
            $event = new Event;
            $event->user_id = $userEvent;
            $event->title = $request->title;   
            if($event->save()){
                $event->coins()->attach($request->coin);
                $event->categories()->attach($request->category);
                return response()->json([
                        'status' => 'Muy bien!',
                        'msg' => 'Tu evento ya fue creado con éxito.',
                        ], 200);
            }
            else {
                return response()->json([
                            'status' => 'Error!',
                            'msg' => 'No pudimos crear tu evento.',
                        ], 401);
            }
       }
}

I think the problem may be when I assign the values to the coin () -> attach () and category () -> attach () section, but I have no idea how to solve it due to my inexperience in the tool.

The system was made entirely in Laravel and it worked without problems, now that it is being updated with Vue it began to give inconveniences.

Any ideas? I occupy Laravel 5,6, Vuejs 2, Axios and Vue-multiselect 2

via Chebli Mohamed