my Mac is not allowing me to erase all content

Got an old Mac from my work just to have, my boss recommended erasing all the content to give it a fresh start and I'm having trouble figuring this out. This is currently the steps I'm taking, System Settings > General > Transfer or Reset > Click "Erase All Content and Settings" - and I receive a message that says Erase Assistant is not supported on this Mac.

I've gone about the steps to sign out of everything (iTunes, Messages, iCloud); I've even followed another community support that said to create a new Admin account and delete the old one. I've followed multiple YouTube videos saying to restart the computer, hold command + R until the apple logo shows to display the Recovery Mode, but this "mode" does not appear and I am brought to the log in screen.

Please advise, thank you!