Print through the locally installed application of Google drive
The best way to upload a file from your google drive for printing is from the locally installed version of google drive. If you are unsure how to install that you can find an install guide by google here.
If you wish to upload a document from google drive and you do not have Google Drive for Desktop installed, you should first download the document you are trying to print. Once this has been completed you can upload it again to the Princh web app as you would with any other document.