When you open the Build Assemblies window and click Customize, the Automatically build required subassemblies check box disappears.
If you want QuickBooks to automatically build your subassemblies, build the assembly prior to customizing the template.
This issue has been resolved in the latest release for QuickBooks Enterprise Solutions 14.0.
Update to the latest release:
Go to the Update to the Latest Release web page.
IMPORTANT: Make sure your product is selected. If not, click the Change link and select your QuickBooks product.
Select and follow the instructions for one of the update options: