Setup the VM

https://github.com/GammaG/macos-virtualbox

Recovery Mode

Enter recovery mode

While beeing inside MacOS enter in terminal

sudo nvram "recovery-boot-mode=unused"
sudo reboot

This sets a firmware variable in nvram indicating that you want to start in Recovery mode on the next boot, and then reboots the machine.

Reboot into system

When done in Recovery mode, run the following from the Terminal in Recovery mode:

nvram -d recovery-boot-mode

This deletes the firmware variable so that the next boot is a normal boot. If Recovery boot fails and you cannot progress, you could also remove the firmware variable by holding down the keys Command, Option, P, and R during boot. This resets the nvram and thus the firmware variable.

Disable Kernel Protection

Deactive SIP

In Recovery Mode Terminal

csrutil disable

to check

Test Protection
csrutil status 

Should show DTrace Restrictions: disabled if not repeat

Deactivate Gatekeeper

In System Terminal

sudo spctl --master-disable

to check

spctl --status

Add Guest Additions Signature

This has to be added in the Recovery Terminal in order to add the signature to the system.

Allow VirtualBox
spctl kext-consent add VB5E2TV963

To check

spctl kext-consent list

Make the folder writeable in MacOS

sudo mount -uw /

sudo chown :admin /System/Library/Extensions/

sudo chmod 775 /System/Library/Extensions/