Data truncated for cloumn

I'm trying to save multiple values of my checkbox, but I'm getting this error

SQLSTATE[01000]: Warning: 1265 Data truncated for column 'category_id' at row 1 (SQL: insert into `category_product` (`category_id`, `product_id`) values (6,5,4, 15))

I'm not sure where I went wrong in getting this to work.

My Product.vue

<template>
    <div class="content-form">
        <form @submit.prevent="submit(data)" enctype="multipart/form-data">
            <!-- BEGIN CATEGORY MODAL -->
            <div class="category-popup">
                <div class="btn btn-info btn-sm mb-15" data-toggle="modal" data-target="#myCategories">
                    Categories
                </div>

                <div class="modal fade" id="myCategories" tabindex="-1" role="dialog" aria-labelledby="myCategoryLabel" aria-hidden="true">
                    <div class="modal-dialog">
                        <div class="modal-content">
                            <div class="modal-header">
                                <div type="button" class="close" data-dismiss="modal" aria-hidden="true">
                                    &times;
                                </div>

                                <h4 class="modal-title" id="myCategoryLabel">
                                    Categories
                                </h4>
    
                                <span>
                                    Please select categories
                                </span>
                            </div>

                            <div class="modal-body">
                                <div class="category-select">
                                    <ul >
                                        <li v-for="category in categories" >
                                            <input type="checkbox" tabindex="1" name="category_id[]" :value="category.id" v-model="category_select">
                                            {{ category.title }}
                                        </li>
                                    </ul>
                                    category modal
                                </div>
                            </div>

                            <div class="modal-footer">
                                <div type="button" class="btn btn-info" data-dismiss="modal">
                                    Save
                                </div>
                            </div>
                        </div>
                    </div>
                </div>
            </div>
            <!-- END CATEGORY MODAL -->

            <!-- BEGIN SUBMIT BUTTON -->
            <input type="submit" class="btn btn-primary" value="Submit">
            <!-- END SUBMIT BUTTON -->
        </form>
    </div>
</template>

<script>
    export default {
        props: [
            'categories',
            
        ],

        mounted() {
            console.log('Component mounted.');

        },

        data() {
            return {
                category_select: []
            };
        },

        methods: {

            save(){
                var formData = new FormData();

                console.log('this is the checkbox - '+this.category_select);

                formData.append('category_id', this.category_select);

                if(this.model.id){
                    formData.append('_method', 'PUT');
                    return axios.post('/admin/products/'+this.model.id, formData);
                }else{
                    return axios.post('/admin/products', formData);
                }
            },

            submit(){
                this.save()
                    .then(
                        function(response)
                        {
                            console.log(response.data.redirect);
                            window.location = response.data.redirect;
                        }
                    )
            },
        }
    }
    
</script>

my ProductsController@store

public function store(Request $request)
{
    $product = new Product();

    $input = $this->getSafeInput($request);

    // Creates an array with the input values and addes the category_id value to the array
    $categoryId = (array) array_get($input, 'category_id');

    $product->fill($input);

    $product->save();

    $product->category()->sync($categoryId);

    return ['redirect' => route('products.index')];
}

I've racked my brain on this and I can't see where I went wrong.