FortiSandbox provides a solution to protect against advanced threats and ransomware for companies who don’t want to implement and maintain a sandbox environment on their own.
Custom VM might fail to activate if custom VM is not configured properly or does not meet the requirements.
This article provides check list to investigate it.

-Run Virtual box and check the following settings:
Make sure Network 'adapter type' is set as per admin guide for the respective Operating system of custom VM requirement

Disable Audio driver as described in the admin guide:
Go to Audio, and uncheck the Enable Audio checkbox.
Disable USB driver as described in the admin guide:
Go to USB, uncheck the Enable USB Controller checkbox.

-Run the custom VM in Virtual-Box and make sure that Windows and Office are activated.
Example: For windows 10, Settings -> Update & security and Activation

-Disable UAC and firewall.
-Check if the Ethernet interfaces are in right order, and change the network adapter names to eth0 and eth1.
-Swap eth0 and eth1 order if necessary.
-Assign data drive to any letter other than 'D', if there is any.
-Enable Administrator account and clear the password
-autorun.bat is under 'Startup for All Users' folder. It's the starter to load launcher.bat in drive 'D', which is a CD-ROM drive that mounts an ISO file.
'\ProgramData\Microsoft\Windows\Start Menu\Programs\Startup'
     @echo off
     if not exist d:\launcher.bat (
     echo Wait for d:\launcher.bat
     rem sleep 5
     ping -n 5 >nul
     goto checker)

start d:\launcher.bat
-Get the VM UUID in Windows: 
mic csproduct get "UUID
-Shutdown the VM and exit VirtualBox program.
Get the Hardware UUID and MD5 checksum of the VM.
Replace the VM name to fit your requirement:

Example for windows WIN7X86VM
vboxmanage showvminfo $VM | grep "Hardware UUID:"
md5sum $VM
-Make sure following value matches:
wmic csproduct get "UUID"
vboxmanage showvminfo $VM | grep "Hardware UUID:"MD5SUM
- Customized image is too big that system doesn’t have enough memory. However, usually sandbox will show up a warning message and block the big clone number. User can reduce its size with Windows Disk Defrag tool.