Bolt - Fatal Error

Configuration autodetection failed because The file vendor/autoload.php doesn't exist. Make sure you've installed the required components with Composer.

This is a fatal error. Please fix the error, and refresh the page. Bolt can not run, until this error has been corrected.
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Fatal error: Uncaught Bolt\Exception\LowlevelException in /homepages/6/d589371217/htdocs/app/bootstrap.php:46 Stack trace: #0 [internal function]: Bolt\{closure}() #1 /homepages/6/d589371217/htdocs/app/bootstrap.php(70): call_user_func(Object(Closure)) #2 /homepages/6/d589371217/htdocs/index.php(46): require_once('/homepages/6/d5...') #3 {main} thrown in /homepages/6/d589371217/htdocs/app/bootstrap.php on line 46