Administrator account but can't erase hardisk

Hi

Its been very puzzling to me. I log in to my iMAC as an administrator but for some strange reason, I can change the privilege of other users. I want to erase and reformat my hardisk but when I am at Disk Utility, I can t choose erase at all.

When I go to finders and highlight my hardisk, click on Machintosh hardisk info, it says I have only read access to the hardisk. I click on the lock at the bottom of the window, prompted to key in my password which I did and try to change to read write access, I was given the message that I don't have the permission.

My OS is High Sierra currently.

I hope somebody can help me to resolve this mystery which I have never encountered. As an administrator, I used to be able to just use disk utility and click on the erase option but now I cannot do that at all.

Appreciate all your help on this. Thank you.