Hi Tom
I believe the print.css has been deprecated in Joomla 4. In Joomla 3, the print.css was loaded when you selected print from the article options (was displayed along with edit and 'send to a friend') however I believe this option has been removed in Joomla 4.
Assuming you are looking to style the page when you print it, you can use the print media query instead...
@media print {
// your CSS goes here
}
Ciaran