I can't save values ​​via POST - laravel vue

I'm trying to save the data that I send from the Event.vue view to the EventController controller's storeEvent method but it gives me a 422 error and I can't find the problem so far.

The Event model has a many-to-many relationship with the Categories model, and also Event has a many-to-many relationship with the Coins model, it is for this reason that I use 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>
 </template>
 <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() sections, but I have no idea how to fix it due to my inexperience with the tool.

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

Any ideas? I use Laravel 5.6, Vuejs 2, Axios and Vue-multiselect 2