Hello @George Goodwin
The most common problem for this might be with the local administrator password being set improperly. The local administrator password in the VM OS must be blank / empty / clear.
If you join the VM to the domain before making a template from it, then the domain security policy gets imposed on the OS in the VM, then a blank local admin password cannot be used or the local admin password specified at deployment does not meet complexity.
Here is an thread as well to help you out with this issue https://social.technet.microsoft.com/Forums/lync/en-US/b3552470-df2e-4700-9c94-cbd952bb44c1/virtual-machine-creation-stuck-at-16-customize-virtual-machine-windows-could-not-apply-unattend?forum=virtualmachinemanager
Hope this resolves your Query!!
--If the reply is helpful, please Upvote and Accept it as an answer--