When performing any action on a PDF form, the following message may appear:
At the top of the form, in yellow, click the "Options" dropdown:
Either option (trust the document once or always) will allow the PDF to be processed.
The tool will prompt you to save the PDF again when you select one of the two options. It will close and reopen again, and if we now re-run the process that was giving the error before it will work.