First of all, please try saving the invoice in different formats.
Just select the format from the "file type" drop-down list. The RTF format is almost guaranteed to work.
As for the error itself, please try the following:
In Microsoft Word, go to "File > Options > General" and uncheck the box "Open e-mail attachments and other uneditable files in Reading view".
If the problem persists, please go to "File > Options > Trust Center > Trust Center Settings > Protected View" and disable all options there.
If the problem still persists: Please try repairing (launching the Microsoft Office installer in "Repair" mode), or uninstalling and reinstalling your current version of Microsoft Office.