macOS System Integrity Protection is Enabled for filesystem protection.
This means the required xxd and dd programs won't be permitted to run to gather the necessary details for the boot sectors of the main system disk (though it may still run on High-Sierra). Other disks should be able to be scanned though.
If you wish to run this dump then you can do one of the following, then reboot:
1 - Disable security by booting in to the Recovery HD and using the Security Configuration utility.
2 - Change bootloader csr-active-config settings to enable CSR_ALLOW_UNRESTRICTED_FS bit.
3 - Boot in to a macOS version before 10.10.4 and change the nvram variable csr-active-config.
You can then boot back in to this macOS system and run the dump.
* Don't forget to reverse this change after performing the dump.