This is the head of the XML file I am using
    <?xml version="1.0" encoding="UTF-8"?>
    <sbml xmlns="http://www.sbml.org/sbml/level3/version1/core" level="3" version="1">
      <model metaid="metaid0" id="bind2_sbml">
        <listOfUnitDefinitions>
          <unitDefinition id="second_order_rate">
            <listOfUnits>
              <unit scale="0" kind="litre" multiplier="1" exponent="1" />
              <unit scale="-6" kind="mole" multiplier="1" exponent="-1" />
              <unit scale="0" kind="second" multiplier="1" exponent="-1" />
            </listOfUnits>
I tried using /sbml[@xmlns="http://www.sbml.org/sbml/level3/version1/core"]/model
as suggested by https://xmltoolbox.appspot.com/xpath_generator.html. But this doesn't work as can be checked using https://codebeautify.org/Xpath-Tester
 
    