Hello there, @blandbooks.

Let's perform some troubleshooting steps to check if this error is caused by data problems in your company file. To start, let's create a sample company file and create a custom template from there. Let me guide you how. 

In your QuickBooks Desktop (QBDT) software: 

1. Go to the File menu. 
2. Select New company. 
3. Select Express Start. 
4. Follow the onscreen steps to finish the setup. 

Here's an article you can read for more details: Create a QuickBooks Desktop Company File. 

If you're able to create a custom template and doesn't revert back after sending it, then we'll have to run the Verify and Rebuild Data tools. This way, it'll resolve minor data issues within your company file. Here's how.

To Verify: 

1. Go to the Window menu and select Close All. 
2. Then, go to the File menu. 
3. Select Utilities. 
4. Click Verify Data. 

To Rebuild: 

1. Go to the File menu. 
2. Select Utilities. 
3. Click Rebuild Data. 

I've got you this article for more detailed steps: Verify and Rebuild Data in QuickBooks Desktop.
 
However, if the issue persists while using a sample company file, I suggest running a clean install. A clean install fixes issues that keep you from opening your company file and a few other situations. 

Also, in case you want to learn how to remove a credit from an invoice, you can refer to this article for the detailed steps: Remove or Unapply a Credit from an Invoice or Bill.

Should you have other concerns or questions, you're always welcome to post here anytime. I'm always here to help. Have a great day!