Unable to use library from composer

Hello,

I have a problem with my composer library.
I just start to use composer so, I made the setup and I have in my Application folder the composer.json, the composer.lock, I installed the phpspreadsheet library. All the files are in the vendor folder.
Finally, I change my config.php with the autoload composer to TRUE.

I have always the same error :

Code:

Message: Class 'Spreadsheet' not found

What have I forgotten ?
Thanks fot the help.

François