The erase option of the hard drive is not highlighted on iMac

You cannot erase the disk you are booted form, so - depending on the model/year of your Mac and OS - you need to either have the original install disks or you can use recovery to boot into and erase your drive.

Just out of curiosity: why would you download an app to format an sd disk? Disk Utility is already built in. And that works great as long as you are not booted into the disk you are trying to erase. Besides, Macs do not have internal SD cards.

If you are preparing this old Mac for sale, trade or recycling, please follow the guidance in this support article:

What to do before you sell, give away or trade in your Mac - Apple Support

Just download and install the app. You will be able to erase the flash drive to factory.

https://www.sdcard.org/downloads/formatter/sd-memory-card-formatter-for-mac-download/