Can not factory reset?

Starting with an apology, as I’m not very good with this stuff. I’m getting rid of my Mac (my info says it is an iMac 21.5” and version macOS14.6.1) and trying to factory reset it, but it doesn’t seem to work. It gives me a message that the erase assistant does not work on this model. When following instructions to open disk utility on startup, it does not show me “Macintosh HD”. Instead, it shows “macOS base system”, and the “erase” button is lower opacity and does nothing when clicked. What do I do from here? Thank you in advance.