Disk Erasing

See if the following article can help: How to erase your Intel-based Mac - Apple Support

You may also find the following helpful:

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

You have to startup in macOS Recovery Mode or from another macOS drive to erase the iMac's Startup Volume.

Please study the "How to erase your your Intel-based Mac" support article, that Tesserax posted above.

or simply click on > > https://support.apple.com/en-us/HT208496 < <

You need to be booted to the recovery partition or Apple's Internet Recovery system to proceed. Follow the instructions in section 7 of the second link Tesserax posted.

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