At the time of executing this command - composer require barryvdh / laravel-dompdf en laravel 5.6 I get the following

Apparently the error is associated with the gd extension; you must open your file php.ini and remove the ; of the following extension to uncomment

This way ;extension=php_gd2.dll should be like this extension=php_gd2.dll

Based on this response from the site in English.