I am trying to parse this XML in PL/SQL
I think there are two things you have wrong here:
Firstly, your XPath expression //@GovernmentCode/ is wrong. It should not have a trailing /, and you don't want the @ either because GovernmentCode is an element, not an attribute.
Secondly, you must specify the XML namespace declaration in your call to EXTRACTVALUE as well as in your call to EXTRACT.
Making these changes leaves you with the following code. I gave it a quick test, and it seemed to work:
SELECT EXTRACTVALUE (VALUE (xml_list), '//GovernmentCode', 'xmlns="http://www.irs.gov/efile"') AS SysID
INTO lv_transid
FROM TABLE (
XMLSEQUENCE (
EXTRACT (in_xmlclob, '/AckTransmission/Acknowledgement',
'xmlns="http://www.irs.gov/efile"'))) xml_list;
You can use regular expression to replace xmlns to empty string... then you can read the xml column
xml:= regexp_replace(xml,'(xmlns:[a-z]|xmlns).*?(\"|\'').*?(\"|\'')',' ');