Laravel 9 error running PHP Unit tests fails with error json not found

I have created the following unit test:

<?php

namespace Tests\Unit;

use PHPUnit\Framework\TestCase;

class DistrictKindTest extends TestCase
{
    /**
     * Index method unit test.
     *
     * @return void
     */
    public function test_index()
    {
        $response = $this->json('GET', '/api/distinct-kinds');
        $response->assertStatus(200);
    }
}

Then I run command: vendor/bin/phpunit --coverage-html tests/coverage I got the following errors:

PHPUnit 9.6.3 by Sebastian Bergmann and contributors.

E....                                                               5 / 5 (100%)

Time: 00:00.151, Memory: 20.00 MB

There was 1 error:

1) Tests\Unit\DistrictKindTest::test_index
Error: Call to undefined method Tests\Unit\DistrictKindTest::json()

/var/www/html/tests/Unit/DistrictKindTest.php:23

ERRORS!
Tests: 5, Assertions: 13, Errors: 1.

What should I use to fix this error?
Maybe I have to add use of some class to fix this error?
Could you advice me on how to fix it?

This type of testing is Release testing. Unit tests are for testing specific classes, not accessing complete routines.

Try using the TestCase used by Release tests:

use Tests\TestCase;