DIAGNOSIS The scanner unit (the whole top part) has a communication fault with the main printer logic. This can happen after a firmware update or if the flat cable connecting the two gets a poor connection.
FIX Perform a full reset. Turn the printer off. Unplug the power cord from the wall. Wait for 60 full seconds. While it's unplugged, press and hold the power button on the printer for about 15 seconds to discharge any residual power. Plug it back in and turn it on normally.
IF THAT DIDN'T WORK Reinstall the scanner driver software. Download the latest 'Epson Scan 2' driver and utilities from the official Epson support page for the WF-4830, uninstall the old one from your computer, and install the fresh download.
HEADS UP If the scanner glass is very dirty, the calibration can fail. Gently clean the glass and the white strip underneath the cover with a soft, dry cloth before trying the reset.