This is one of those situations where it's better to target the Post model directly instead of the Postcategories model. You could probably do this with a join instead of using relationships.

$posts = Post::select('posts.*')->join('postcategories', 'postcategories.id', '=', 'post.categories')->where('postcategories.slug', $category)->paginate();

I'm not sure if I got your table names right. But hopefully you get the idea.

As far as I understood you want to bring all posts from a certain category, right?

So try this:

$posts = Post::whereHas('catposts', function($query) use($category)
{
   $query->where('slug', '=', $category);
})->paginate(1);