What am i missing here? subscribers/bulk

There are multiple errors here, for a start you're sending a json blob when the api accepts form data only, this is why you should use the API Client we provide.
So start from there ;)
Thank you for your answer, I appreciate it. I have one more question: How can I perform Confirmed uploads without sending emails to users?
You can add status confirmed if you want to force confirmed status without confirmation email to be sent:

$response = $endpoint->createBulk('LIST-UNIQUE-ID', [
        'EMAIL'    => 'john.doe@doe.com',
        'FNAME'    => 'John',
        'LNAME'    => 'Doe',
        'details' => [
            'status' => 'confirmed',