Can not upgrade my OS X 10.9.5

Perhaps your Mac hardware could accept one of the later macOS upgrades

from their web servers; w/downloads and various steps to follow. ~Maybe

first attempt newer El Capitan 10.11.6. ~ Then try another one, more recent?

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

"IF your Mac isn't compatible with the latest macOS, you might still be

able to upgrade to an earlier macOS, such as macOS Catalina, Mojave,

High Sierra, Sierra, or El Capitan."

Older Mac OS X is on 'final step' of Mavericks 10.9.x ~ not Classic OS9.

Lesser 'updates' do not change main #. keep your software up to date.

You may be able to use link to El Capitan with special instructions. Or

use: • How to create a bootable installer for macOS - Apple Support

First part of this suggests to get a downloadable installer for El Capitan

that then has special instructions to use; if that doesn't work, read more.

{..Details are like mushrooms, among forest flora. ~ Not all are edible..}

Also, when posting it is extremely helpful if you tell us the year of the iMac. You can find this in About This Mac which you can find by clicking the Apple symbol in the upper left of the display.