snipe-it/app/Http/Requests/AssetCheckinRequest.php

<?php

namespace App\Http\Requests;

class AssetCheckinRequest extends Request
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        $settings = \App\Models\Setting::getSettings();

        $rules = [];

            if($settings->require_checkinout_notes) {
            $rules['notes'] = 'string|nullable';
        }
        return $rules;
    }

    public function response(array $errors)
    {
        return $this->redirector->back()->withInput()->withErrors($errors, $this->errorBag);
    }
}