how do I erase my mac mini before returning it

I'm not sure where "enter" or the sign in screen is involved with step 5 of What to do before you sell, give away, or trade in your Mac - Apple Support . You press the 4 keys immediately after powering on. I would try holding two fingers lightly touching but not pressing the option and command keys with one hand, hit the power key and immediately press those keys and use the other hand for the P and R.

See the following from Reset NVRAM or PRAM on your Mac - Apple Support

Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R. You can release the keys after about 20 seconds, during which your Mac might appear to restart.

for a new 2020 M1model, see > https://support.apple.com/en-us/HT212030

for a new Intel model, see > https://support.apple.com/en-us/HT208496

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