Why isnt Macintosh HD showing up in disk utility?

My mom recently upgraded to the 2020 macbook pro 13", and gave her old 2014 retina model to me. When I went to wipe it, Macintosh HD doesn't show up in disk utility. The computer still functions normally and I am still able to logon and everything. Is there a solution? talked to apple support and they told me to take it to a technician near me just to squeeze more money out of me. The mac is running High Sierra.