How to erase the system disk on iMac or MacBook?

User have to disable system integrity protection to be able to erase the system disk.

How to turn off System Integrity Protection in macOS:

1. Click the Apple symbol in the Menu bar.
2. Click Restart…
3. Hold down Command-R to reboot into Recovery Mode.
4. Click Utilities.
5. Select Terminal.
6. Type csrutil disable.
7. Press Return or Enter on your keyboard.
8. Click the Apple symbol in the Menu bar.
9. Click Restart…

For more information about System Integrity Protection on Mac, read here.