MacBook Pro keeps saying my printer is idle

one very interesting feature of AirPrint is that it may OR MAY NOT use your existing Wi-Fi network to make the connection. It may try to connect directly, so your Printer must be close enough to get the signal directly.

otherwise your Printer need to know three things:

what network-name to join

what encryption method and password

how it should obtain an IP address, manual or DHCP

you may need to re-enter those from the printer control panel or use HP utilities like HP smart App and/or HP easy start to find the Printer.

That appears to be a printer capable of USB connection or Wireless direct printing using AirPrint driver.

https://support.hp.com/us-en/product/hp-envy-4500-e-all-in-one-printer-series/5304875/model/5368518/document/c03755986

it appears to be supported in MacOS from 10.6 Snow Leopard through 10.11 El Captain. That suggests its driver is not 32-bit capable, and to continue to use it, you will need to use AirPrint and wireless direct.

https://support.hp.com/us-en/drivers/selfservice/hp-envy-4500-e-all-in-one-printer-series/5304875/model/5368518

Idle is merely the assumed default state.

You see Idle when you are not talking to the printer the way it expects to be talked to, and/or it stops responding.

if you have recently upgraded MacOS, you may need to download a driver, if available, and/or set up the Printer again.