Failing to connect to Microsoft Bookings API

First of all, I can recommend the PHP package https://packagist.org/packages/microsoft/microsoft-graph for the usage of the Microsoft Graph API. It's easier than writing all the requests on yourself with Guzzle.

I'm confused that you are trying to use your username with password and a client secret at the same time. Client secrets have the advantage that you don't need to use (and therefore expose) your account password compared to a simple app token with strictly limited permissions. So you should try to get an access token with client ID, client secret, scope and grant type, for example like this:

$guzzle = new \GuzzleHttp\Client();
$url = 'https://login.microsoftonline.com/' . $tenantId . '/oauth2/v2.0/token';
$token = json_decode($guzzle->post($url, [
    'form_params' => [
        'client_id' => $clientId,
        'client_secret' => $clientSecret,
        'scope' => 'https://graph.microsoft.com/.default',
        'grant_type' => 'client_credentials',
    ],
])->getBody()->getContents());
$accessToken = $token->access_token;