Esri Python script to edit multiple metadata elements failure

import os
import sys
import xml.etree.ElementTree as ET
import arcpy
from arcpy import metadata as md

# Script arguments...
Source_Metadata = arcpy.GetParameter(0)

# Local variables
#    new purpose text
newPurpose = "This is new text for an existing Purpose metadata element."
newCredits = "This is text for a new Credits metadata element."


# function to update purpose and credits in metadata
def update_metadata(root):
    num_elements = 0

    # modify purpose element's text
    # there is only supposed to be one purpose element in metadata
    # replace purpose element text if element exists
    # if element doesn't exist, do nothing
    purposeEls = root.findall(".//idPurp")
    for element in purposeEls:
        if element.text is not None:
            element.text = newPurpose
            num_elements += 1

    # add credits element to dataIdInfo parent, if the parent exists
    # ISO allows many dataIdInfo groups; ArcGIS supports only one, so get the 1st
    # ISO allows many idCredit elements, and many are supported in ArcGIS
    # append a new idCredit element with appropriate text, existing elements remain
    dataIdInfoEls = root.findall("./dataIdInfo[1]")
    for element in dataIdInfoEls:
        if element:
            newCreditEl = ET.SubElement(element, "idCredit")
            newCreditEl.text = newCredits
            num_elements += 1

    return num_elements


# get and save item metadata
for item in Source_Metadata:
    arcpy.AddMessage("Item: {0}".format(item))
    
    # get the item's metadata xml
    item_md = md.Metadata(item)
    metadata_xml_string = item_md.xml
    
    # create an ElementTree object from the metadata XML string
    root = ET.fromstring(metadata_xml_string)

    # call the update_metadata function to modify the item's metadata
    changes = update_metadata(root)
    
    if changes > 0:
        # get modified XML
        updated_xml_string = ET.tostring(root, encoding="utf-8")
        
        # import result back into metadata 
        arcpy.AddMessage("Saving updated metadata with the item...")
        item_md.xml = updated_xml_string
        item_md.save()
    else:
        arcpy.AddMessage("No changes to save")


arcpy.AddMessage('Finished updating metadata for all source metadata items')

Trying to take this Esri-sourced script to update multiple metadata elements. If I run this in a standalone script without editing, it doesn’t do anything, no errors. If I take the script and create a script tool in a GDB inside of ArcGIS Pro, it fails when I select the XML file as a parameter. What do I need to do to get this script to work on my metadata XML?

The script (meant for ArcGIS Pro) was sourced from:
https://pro.arcgis.com/en/pro-app/arcpy/metadata/migrating-from-arcmap-to-arcgis-pro.htm#ESRI_SECTION1_098BC26422304FAE96A19FB4B6528E07