I'm trying to securely erase my SSD by using diskutil in terminal in the recovery mode (restart --> hold down command+R --> terminal)
I first use diskutil list to find the disk name, which is disk0:

Then I use diskutil secureErase 0 disk0 to erase the disk:

However, the error "Couldn't open disk" happened. I tried to go to the UI version of Disk Utility, unmount the disk and erase again, but the same problem happens.