Explanation of this error

This is a security feature of Laravel. It is designed to protect you against form manipulation when using mass assignments.

For example on a sign-up form: When you have an is_admin column in your database, a user simply could manipulate your form to set is_admin to true on your server, and therefore in your database. This security feature prevents that by using a whitelist to define safe fields.

How to fix that

You need to set a $fillable property on your model. It's value must be an array containing all fields that are safe to mass assignable (like username, email address, ...).

<?phpnamespace App;use Illuminate\Database\Eloquent\Model;class Comment extends Model{    # This property!    protected $fillable = ['body'];    // ...}

See "Mass assignment" in the docs:https://laravel.com/docs/5.5/eloquent#mass-assignment

Mass assignment is when you send an array to the model creation, basically setting a bunch of fields on the model in a single go, rather than one by one, something like what you did here:

public function addComment($body){    $this->comments()->create(compact('body'));}

You need to add the field you are populating to the fillable array in Comments.php model:

<?phpnamespace App;use Illuminate\Database\Eloquent\Model;class Comment extends Model{    protected $fillable = ['body'];    public function post()    {        return $this->belongsTo(Post::class);    }}

As the documentation states:

You may also use the create method to save a new model in a single line. The inserted model instance will be returned to you from the method. However, before doing so, you will need to specify either a fillable or guarded attribute on the model, as all Eloquent models protect against mass-assignment by default.

Hope this helps you.