You make no mention of your Printer model.
Unlike AirPrint, that is driverless printing protocol, the scan function of your printer requires additional software to be usable with your iPad (or iPhone).
To scan documents to your iPad from your multifunction printer, you will need to install a manufacturer-supplied App on your iPad to enable this capability. By example, HP multifunction printers use the HP App (previously known as HP Smart) that can be downloaded and installed from the Apple App Store:
https://apps.apple.com/app/hp/id469284907
Epson and Brother, among other manufacturers, have similar Apps available.