First as mentioned above your file must be named Post.php and not posts.php.

Since you are using App\Post at the beginning of your file, you can simply call Post::class $factory->define(Post::class, function (Faker $faker)

Another option could also be that you are missing the factories folder in your composer classmap:

"autoload": {
        "psr-4": {
            "App\\": "app/"
        },
        "classmap": [
            "database/seeds",
            "database/factories"
        ]
    },

If this does not solve the problem, then you might need to post your code where you are calling your factory

@edit

this is pretty good overview over conventions

Try publishing your laravel stubs and confirm that the stub file contents are defined as expected.

1. Publish the stubs.

php artisan stub:publish

• This should create a /stubs folder in the root project directory.
• Inside that folder, you will see all stubs.

2. Most specifically, open the stub file called factory.stub

• It's file contents should look something similar to this:

<?php

namespace {{ factoryNamespace }};

use Illuminate\Database\Eloquent\Factories\Factory;
use {{ namespacedModel }};

class {{ factory }}Factory extends Factory
{
    /**
     * The name of the factory's corresponding model.
     *
     * @var string
     */
    protected $model = {{ model }}::class;

    /**
     * Define the model's default state.
     *
     * @return array
     */
    public function definition()
    {
        return [
            //
        ];
    }
}

Notes:

From the looks of it, it appears as if your current factory stub is missing the section(s) below:

// ...
use {{ namespacedModel }};
// ...

    /**
     * The name of the factory's corresponding model.
     *
     * @var string
     */
    protected $model = {{ model }}::class;

// ...

Ideally speaking, in normal (default) cases, running a command that generates a factory with a linked model should look like this:

Command:

php artisan make:factory BoardFactory --model=Board

Expected Output File (database/factories/BoardFactory.php):

<?php

namespace Database\Factories;

use App\Models\Board;
use Illuminate\Database\Eloquent\Factories\Factory;

class BoardFactory extends Factory
{
    /**
     * The name of the factory's corresponding model.
     *
     * @var string
     */
    protected $model = Board::class;

    /**
     * Define the model's default state.
     *
     * @return array
     */
    public function definition()
    {
        return [
            //
        ];
    }
}

Addendum:

As pointed out in a comment by @miken32, in Laravel versions published later than Oct 22, 2021, declaring a model attribute in your Factory class will no longer be necessary:

Laravel Pull Request

At this time, database factories have this hidden feature where database models can be "guessed".

So, this pull request proposes that remove protected $model from the Factory stub, as probably the current "guess" logic works for 99.99% of the people. In addition, I've also pull requested to the skeleton that we remove protected $model = User::class from the UserFactory.php: laravel/laravel#5713.