Authorized .Net payment gateway not working

Hello sir, I have implemented authorized.net payment gateway in my project. When i am running my project, It is saying -

Warning: require(vendor/autoload.php): failed to open stream: No such file or directory in C:\xampp\htdocs\sample-code-php-master\PaymentTransactions\charge-credit-card.php on line 2

Fatal error: require(): Failed opening required 'vendor/autoload.php' (include_path='.;\xampp\php\PEAR') in C:\xampp\htdocs\sample-code-php-master\PaymentTransactions\charge-credit-card.php on line 2

I have already intalled the composer, but It's not helping me to solve the error .
So how can I Solve this problem ?