Hi Philippe,
I advise against of doing what I will propose next especially if you are not experienced user and because it will make harder the maintenance of the website.
Now the Entity Print module requires the PDF Generator module in order to work. Mount the filesystem of your website and go into the PDF Generator module’s folder. Create a lib directory and copy dompdf files there. Then open the pdf_generator.module file with an editor and add the following line at the top after the <?php and use Drupal\Core… lines:
require_once(‘lib/dompdf-dompdf/v0.8.3/autoload.inc.php’);
The above path might differ depending which version of the dompdf library you have.
Then save the file. Go to your website and clear caches and everything should be fine.
In the future when you have to update the PDF generator module to a newer version you must do that manually. Download the module locally, make the same change to the pdf_generator.module file, mount your website’s filesystem, delete the pdf_generator module from your website and copy the local module in its place. Then clear caches of your website again. You might also need to check for database updates by going to https://.web.cern.ch/update.php