erasing hard drive, to sell / donate

I have an old late 2012 27” Imac, that I would like to wipe clean, before giving it away. I followed instructions to hold the Command + “r” keys down, during startup, but that is not working….and just starts it up to my normal home screen . I cant figure out how to start into disk utility mode, in order to wipe my computer…. I’m pretty knowledgeable, and have wiped mac’s many times, so I’m not sure what is going on here…. Any suggestions?