If a document is in "read only" mode, Woodpecker won't be able to insert fields, save any fields, or populate the document.

The simplest thing to do is to either click the "duplicate" button that Word presents when showing the "Read only" mode warning message, or simply do a "File" - "Save as" and save the document as a new document.

Open up your new document with Woodpecker and you'll be good to go!