When RootsMagic creates a report, it needs to access a printer driver to obtain information like page size, etc. This means you need to have a printer driver installed, even if you don't have a printer installed.
Go into your Windows Control Panel and choose the Printer settings. If there is a printer driver already installed make sure it is selected as the default printer. If there isn't a printer installed, choose "Add a printer" and select a printer (you can choose almost any printer... you might choose one that you may already own but is on a different computer). After the printer is added, make sure it is selected as the default printer.