While we're unable to print a journal entry voucher in QuickBooks Online, you can use the universal keyboard shortcuts to print a basic version of any web-page you're using. I'll guide your how.
Navigate to the Journal Entry you'd like to print and open it.
Press CTRL+P on a Windows PC or Command+P on a Mac.
Your browser's Print Preview window will open. Certain browsers like Google Chrome allow you to change the Orientation (Landscape vs. Portrait) and the Scale of the printed page to ensure all information fits.