What does this error mean after calling the openssl_encrypt function?

When calling the function openssl_encrypt , it is always returned false. When I check the openssl_error_string function, I get the following error:

  

error: 0906D06C: PEM routines: PEM_read_bio: no start line

The code I'm testing is this:

$data = 'Olá, Wallace!';

$encryption_key = openssl_random_pseudo_bytes(32);

$iv = openssl_random_pseudo_bytes(openssl_cipher_iv_length('AES-256-CBC'));

$encrypted = openssl_encrypt($data, 'AES-256-CBC', $encryption_key, 0, $iv);

if (false === $encrypted) {

    var_dump(openssl_error_string());

    exit;
}

Why is this error generated?

I'm doing these tests trying to find a solution to this question:

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