Hello there, @purcell. 

Thanks for visiting the Community today. I'll be with you in fixing this custom templates issue. 

To get rid of the error, I'd recommend managing your invoice template using the sample file in QuickBooks Desktop. This will help us determine the damaged company file. 

Here's how:

1. Click the File menu.
2. Select Close Company.
3. Hit the open a sample file drop-down arrow, then pick a file.

If you can successfully do your templates using the sample file. If that works, go back to your company account and use the Verify/Rebuild utilities from the File menu. It'll help us identify the most commonly known data issues within a company file. 

I've also got you some of our helpful articles about managing invoice templates and fixing common issues when using and customizing templates:

• Use and customize form templates
• Fix common issues when you use and customize templates

I'm always willing to help you some more with any of your additional or other concerns. Take care!