How to get Access Token from gmail?

The Main issue I am facing here is as soon as I hit the sendmail api ( This API send mail using google email api ) it gives me error of Undefined index: access_token

I want to send email using GMAIL Email API in my Laravel Project. For that I had followed this github doc:https://github.com/dacastro4/laravel-gmail. I am successfully able to do setup it with my project. But then struck in above error. Below is my code:

        use Dacastro4\LaravelGmail\Services\Message\Mail;

        $mail = new Mail;
        $mail->to( 'TOMAIL', $name = "Tester" );
        $mail->from( 'MYMAIL', $name = "Own Tester" );
        $mail->subject( "Hello World" );
        $mail->message( "Google It Brother." );
        $mail->send();

Expected response is it should send the email to user using gmail api.

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