I ran across this while building a lab for a customer this week and hopefully this will save you a little time. When I built the vApps in vCloud Director 5.1.2 I selected the "Reset Password" option in the VM Properties as shown below....
However, this setting in a vApp template causes the vCAC vApp deployment workflow to fail with the following error:
Error in workflow cleanup. [Workflow Instance Id=somenumber]
Error customizing vApp.
Inner Exception: The administrator password cannot be empty when it is
enabled and automatic password generation is not selected.
This is a 'feature' of the vCD 5.1.2 API and not a problem with vCAC. It seems that passing the administrator password through the API was removed as of that release. I understand that this is likely to be changed back in a future release - and I'll try to remember to update this post if/when that happens.
For now, you will need to disable the password reset within the VM properties for any template you wish to deploy from a vCAC vApp blueprint.