Hi mimilshea.
In addition to the printer drivers, you will need to download and install the scanner drivers at the D420 Drivers & Downloads page. They are provided in a separate installation package from the printer driver package. If the printer was previously installed before updating to OS 10.15, you may need to remove it from the list of printers or reset the printing system, then add it back.
If you have both Image Capture and the MF Toolbox / MF Scan Utility open at the same time, it's possible for one to interfere with the other; ensure only one piece of scanning software is up and running. If the problem is lingering after a previous scan, before updating the scanner drivers, power off the printer, wait ~ 30 seconds, then power the printer back on. This will normally end any lingering scan processes.
If the D420 is still unable to scan after installing its scanner drivers and power cycling the printer itself, ensure the security software and settings on the Mac is allowing the printer to communicate and interact with the computer. Also check that the USB connection between the printer and computer is not damaged, and replace the cord if it is.
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