There are a few possible causes as to why the Cash Register printer won't print:
From Windows Printers and Scanners, select the printer, and ensure that you are able to print a test page.
Ensure "Enable Receipt Printing" is ticked. You can do this by doing the following:
Open and login to Cash Register.
At the top, click Print.
Click Enable Receipt Printing.
Ensure the Cash Register Number is set up correctly. See: Set up a cash register number.