"The printer's paper size has changed ... Repaginate to the new paper size?"

This main this message pops up is when you move documents between Windows and Mac because the operating systems use different printer page setup parameters.  However, if you get this, the solution is check is to make sure each computer on which you edit the document has its Windows default printers set.  Windows 10 has a habit of un-setting the default printer during updates.  Screenwriter needs a default printer set to get the page size information. 

• The solution is to (re)set the default print driver:Open the Windows Control Panel (you can search for it using Cortana, the Windows search feature)
• Select Hardware and Sound
• Select Devices and Printers
• If you see the Microsoft XPS Document Writer there, right-click on it and set that as the default printer.  This is Windows' generic print driver that also happens to support exporting to PDF.  Alternatively, select the printer you use and set IT as the default printer.  The important point is that a default printer is set.
• In Screenwriter, go to File > Print Setup and make sure the Microsoft XPS Document Writer, or your actual printer is selected.
• Make sure a default page size is selected, such as Letter or A4.
• Press OK.

This should clear up the problem.