I cannot see a particular USB printer from AirPort Extreme
Plug it in via Ethernet, not USB.

Any printer with built-in networking support (Ethernet or Wi-Fi) is best connected to the network via its network port. That's the most natural, best-supported, most reliable, most fully-featured way for your devices to discover and print to it.

When base stations first supported USB printer sharing in 2003, it was kind of a hacky workaround for the printers of that time, which were often USB-only. It's still a hacky workaround today, but luckily most people don't need to use it anymore as most printers (even the free-after-rebate consumer models) added networking by about 2005.