Barcode Substr

Oracle BI Publisher Desktop 12.2.1.4.0

I am attempting to barcode a substring value and am not having much luck

I have tried this:

<?format-barcode:xdofx:substr(Related_PO_SO_WO_Number_Displa_ID20,2,13);'code39'?>

and get this error

Namespace prefix 'xdofx' used but not declared.

This

<?format-barcode:substr(Related_PO_SO_WO_Number_Displa_ID20,2,13);'code39'?>

and get this error:

Parse Error in substr function.
oracle.xdo11g.xpath.XPathException: Parse Error in substr function

This

<?format-barcode:<?xdofx:substr(Related_PO_SO_WO_Number_Displa_ID20,2,13)?>;'code39'?>

I don't get an error but don't get a barcode either