Error code 0x803F7001 activating Windows inside Virtual Box
According to Microsofts help page, the error 0x803f7001 means that no valid license was found:
If you have a valid product key, use it to activate Windows:
Select the
Startbutton, then selectSettings>Update & security>Activation. SelectChange product key, and then enter the 25-character product key.
Follow the steps and enter here your valid Windows 10 key. If you have upgraded your physical Windows PC, the license ONLY applies to the physical PC and not to any new VMs. Here you need to buy a valid license.