Laravel8 tdd: session is missing expected key errors

I just want to test an easy input field but I get this error!

/** @test */
public function email_must_be_a_valid_email()
 {
   $response = $this->post('/api/contacts', array_merge($this->data(), ['email' => 'NOT AN EMAIL']));
   $response->assertSessionHasErrors('email');
 }


private function data()
{
  return [
          'name' => 'Test Name',
          'email' => 'test@hotmail.com',
          'birthday' => '05/14/1988',
          'company' => 'ABC String'
        ];
}

Thses are the Controller and the request rule. I hope u can help me with it.

class StoreController extends Controller
{
    public function store(ContactsRequest $request)
    {
        $data = $request->validated();
        Contact::create($data);
    }
}

public function rules()
    {
        return [
            'name' => 'required',
            'email' => 'required|email',
            'birthday' => 'required',
            'company' => 'required',
        ];
    }

I hope u can help me with it.

Thanks for your tips. I've solved the problem! But this kind of error message does not help at all to find the solution quickly. The solution is that I need to be logged in to create contact and then validate form data. But the error message says something completely different!

protected $user;

protected function setUp(): void
{
   parent::setUp();
   $this->user = User::factory()->create();
   $this->user->createToken(Str::random(30))->plainTextToken;
}


/** @test */
public function email_must_be_a_valid_email()
{
   $this->actingAs($this->user);
   $response = $this->post('/api/contacts', array_merge($this->data(), ['email' => 'NOT AN EMAIL']));
   $response->assertSessionHasErrors('email');
}