Linked below is the Google Drive zip file of the VMDK you’ll need to get High Sierra working on VirtualBox. Watch the video above FIRST so that you can successfully download the file as a whole. For whatever reason if you try to download direct from web browser google drive will chop the file into multiple parts breaking the file.
READ THIS: QUOTA EXCEED ERROR… AFTER YOU “ADD TO MY DRIVE” (1:20 into the video) THERE IS AN ADDITIONAL STEP…. YOU HAVE TO RIGHT CLICK ON THE VMDK IN YOUR DRIVE AND MAKE A COPY… THEN REMOVE/DELETE THE ORIGINAL ONE. AFTER YOUVE DONE THIS PROCEED WITH THE REST OF THE STEPS. SORRY THIS IS A PAIN IN THE NECK.
DOWNLOAD : High Sierra VMDK