got this @rin4ik

array:3 [▼
  "_token" => "aho2Z5aksoHRgQI7zMUjMt3dM0YlSZih8nm5L3bS"
  "post" => "4"
  "tags" => array:1 [▼
    0 => "4"
  ]
]

try these one of them should work

  $post = Post::where('id', $request->get('post'))->get();
  $post->tags()->sync($request->tags);

  $post = Post::where('id', $request->get('post'))->firstOrFail();
    $post->tags()->sync($request->tags);

  $post = Post::find($request->get('post'));
    $post->tags()->sync($request->tags);

Thank you very much @rin4ik and @Snapey. It works now and I gained plenty of knowledge from you guys. Thank you :)

@fromthedark_14 you are welcome.