Run-time error '5': Invalid procedure call or argument when Running Macros in Microsoft Word

This was due to the header having not been added as a merge field. After adding the header, you will need to hit Alt F9 and make sure its a merge field.

If you hit Alt F9 and the header does not toggle to show merge field, then you will need to add each merge field manually and then save the template after. Typically you will not have to do that, but sometimes it is necessary.

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If you have verified that all of the fields in the header show as a merge field when toggling Alt F9 and the issue still occurs, this can be caused by formatting in the template itself such as inserted objects that cause the macro to not be able to read the document. These objects will need to be deleted.

An example of what an object looks like is below. Note the object anchor on the left, the layout properties on the right and in the middle, the main object window  

that was inserted.