How to save In Pivot Table in Laravel

How to save data to pivot table in laravel , I have two tables has many-to-many relationship , so i create pivot table , I tried the sync() method to insert but it keep gives me this exception:

"message": "Call to a member function sync() on null", "exception": "Symfony\Component\Debug\Exception\FatalThrowableError",

here is Contract Model :

class Contract extends Model
{
    public function skills(){

    $this->belongsToMany(RequiredSkills::class , 'contract_skills')->withTimestamps();

    }

}

and here is RequiredSkills Model :

class RequiredSkills extends Model
{
    public function contract(){

        $this->belongsToMany(Contract::class , 'contract_skills');

    }

}

and here is the controller :

class RequiredSkillsController extends Controller
{
    public function store(Request $request){
        $contract = new Contract();
        $data = $request->all();
        $requiredSkills = RequiredSkills::create([
            'contract_id'=>$data['contract_id'],
            'skills_tag'=>$data['skills_tag'],
            'user_id'=>auth()->guard('api')->user()->id
        ]);
        $contract->skills()->sync($requiredSkills->id);

        return $requiredSkills;

    }
}

how to solve the exception ?

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