The error you are getting usually means that another software or another computer on your network has opened the scanner.

If this is not the case, you might have multiple conflicting drivers because the old ones were not correctly uninstalled during the last OS update. You can fix this by performing the following steps:

• If you have installed any custom scanner software that came bundled with your printer/scanner, uninstall it with the provided uninstaller. Please note that in many cases this software is not installed because macOS already comes with the drivers. In this case, you can skip this step.
• Delete the contents of the folder /Library/Image Capture/Twain Data Sources/
• Delete the contents of the folder /Library/Image Capture/Devices/
• Empty the trash and restart macOS
• Start the Image Capture Application that comes with macOS. It will detect the scanner and ask you if you would like to download and install the latest drivers. Confirm that. Wait for the drivers to install, then quit Image Capture.

You should now be able to use the scanner in PDFScanner.

If this does not help, something is wrong with your scanner or your machine, but you need to contact the support hotline of your scanner’s vendor to resolve this as it is not related to PDFScanner.