Here is how you can do it:

1

Select the parts of the document that you would like to be locked in place.

2

Go to Review → Protect → Restrict Editing  (or Developer → Restrict Editing)

Search for Review tab
Search for Review tab
Click on Protect button
Click on Protect button
Click on Restrict Editing
Click on Restrict Editing
3

Under Editing restrictions, check Allow only this type of editing and choose No changes (Read only).

Check the box for Allow only this type of editing in the document
Check the box for Allow only this type of editing in the document
4

Click Yes, Start Enforcing Protection and set a password.

Click on Yes, Start Enforcing Protection
Click on Yes, Start Enforcing Protection
5

Save the document.

Now with these settings applied to your Word document template, you will no longer have issues with the document creation process in InvoiceFlow.