New Pixma Printer/scanner NOT showing the Scanner option when in the Printer/Scanner System Settings

You have not clarified if you installed Canon drivers whether from an (outdated) included DVD, downloaded them, or have configured this networkable TS6420a for use with Apple's built-in AirPrint. The latter requires no Canon software or drivers and allows scanning using Apple's Image Capture, or Preview applications. It is noteworthy that Image Capture on Sequoia has an OCR option to produce searchable PDFs from the scan.

If you are using Canon drivers, you will need an ICA driver to enable its scanner to work with the aforementioned Apple applications.

At this destination, choosing macOS v15 instead of the incorrect default of 10.15, Canon does not offer an ICA driver for this multi-function device on macOS Sequoia. That suggests to me that they expect you to be fully invested in their software tools which is a huge dependency on your part.

I recommend that you configure the TS6420a as either a wired, or wireless network device and then set it up to use Apple's AirPrint which eliminates your Canon software dependency. It would also automatically give you access to Apple's previously mentioned scanning applications and printing functionality.

In System Settings > Printers & Scanners panel, locate Printers and if your TS6420a is listed there, right-click on it and choose Reset print system… If it is not shown, then just click the Add Printer, Scanner, or Fax… button. In either case you will end up on an Add Printer panel where Apple's Bonjour should automatically detect the networked TS6420a. Click that entry on the panel to select it and to activate the Use selection to automatically configure for AirPrint. Then click the Add button and you should have a fully functional TS6420a.

That Add panel looks like this for my Samsung laser that is configured with AirPrint:

You always want a printer with at least AirPrint, and a multi-function with AirPrint and TWAIN, and probably also with IPP/IPPS for best flexibility. Printers and without this support are entirely dependent on vendor drivers.

I don’t see TWAIN listed in the specs for this Canon PIXMA TS6420a printer. AirPrint is listed.

Once connected to the network, check that the printer firmware is updated to current, and test with AirPrint again. See page 162:

https://downloads.canon.com/hsg2023/ijprinters/manuals/TS6400ser_OnlineManual_Mac_EN_V01.pdf

If Canon support cannot get this resolved for macOS 15 on current firmware, and if you cannot get this working with a reset and no vendor drivers and the AirPrint printing (and AirPrint scanning) path, then return for refund or replace the printer, and acquire a printer from a different vendor, and particularly a printer with AirPrint and TWAIN support.

• Print wirelessly from your Mac to your printer - Apple Support
• Reset the printing system on your Mac to solve a problem - Apple Support

Click on the printer in the list. In the dialog that pops up, is there an Open Scanner button? That will open Image Capture (a macOS app, but you could also try opening that directly).