macOS System Integrity Protection is Enabled for unsigned kexts.

This means the pmem.kext cannot be loaded and the memory dump will therefore not work.

If you wish to run this dump then you can do one of the following:
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_UNTRUSTED_KEXTS 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.