OS Mojave cannot see Canoscan 8400f

The drivers for that scanner (and my 8600F) are not compatible with High Sierra and following systems. However, there is a 3rd party scanner driver app, VueScan, that will recognize the scanner and run it very well.

You will need to use the 32 bit version. There's a demo version available to try to see if you like it. The demo version puts a watermark on all scans and will not work with the systems after Mojave (it works with Mojave). The scanner must be turned on before VueScan in launched.

The “no driver” from Canon is telling you that this old scanner is no longer supported by Canon, as they have moved on after ten years. Hamrick's 32-bit VueScan (non-subscription purchase) will drive this scanner for flatbed scans, but not transparency scans which require Canon 32-bit drivers. If you want to continue using this scanner, hook it up to Windows and Canon drivers are probably still available.