I am guessing it is because you haven’t done this: Create a vCenter Server Application to Generate Computer Names and IP Addresses
I had to do the following when it came down to allow customization when creating vm's. Hope this help.
1. Create a role.
2. Give the following permission.
a. VirtualMachine.Provisioning.Modify customization specification
b. VirtualMachine.Provisioning.Read customization specification
3. Apply the new role at your vCenter to the required group but don't propagate down.
That fix my problem.