Can't upgrade to latest MacOS Catalina

A 2011 iMac cannot be upgraded beyond 10.13.6. See the following from. Identify your iMac model - Apple Support

iMac (27-inch, Mid 2011)

Model Identifier: iMac12,2

Part Number: MC813xx/A, MC814xx/A

Newest compatible operating system: macOS High Sierra

You cannot run Safari 15 or 16 with that iMac. For security reasons you should consider using a 3rd party browser that supports running their latest version with High Sierra 10.13.6. Examples include Chrome and Firefox.

A 2011 iMac cannot update beyond macOS 10.13.6 High Sierra, I tracks the hardware to run anything newer.

You can try a different browser such as Brave or Firefox to pay your bills.

Start considering a newer Mac.