Saw this in 4.19.5, but given the code, I believe it also still happens in master.

root@n-ad:~# samba-tool gpo manage motd set {177F310B-5416-48A9-8A39-EA8919EB7964} "Welcome" -U Administrator%Passw0rd
WARNING: Using passwords on command line is insecure. Installing the setproctitle python module will hide these from shortly after program start.

root@n-ad:~# samba-tool gpo manage motd set {177F310B-5416-48A9-8A39-EA8919EB7964} "Welcome" -U Administrator%Passw0rd
WARNING: Using passwords on command line is insecure. Installing the setproctitle python module will hide these from shortly after program start.
ERROR(<class 'UnboundLocalError'>): uncaught exception - cannot access local variable 'data' where it is not associated with a value
  File "/usr/lib/python3/dist-packages/samba/netcmd/__init__.py", line 279, in _run
    return self.run(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/samba/netcmd/gpo.py", line 3829, in run
    text = ET.SubElement(data, 'text')
                         ^^^^

        try:
            xml_data = ET.fromstring(conn.loadfile(vgp_xml))
        except NTSTATUSError as e:
            if e.args[0] in [NT_STATUS_OBJECT_NAME_INVALID,
                             NT_STATUS_OBJECT_NAME_NOT_FOUND,
                             NT_STATUS_OBJECT_PATH_NOT_FOUND]:
                # The file doesn't exist, so create the xml structure
                xml_data = ET.ElementTree(ET.Element('vgppolicy'))
                policysetting = ET.SubElement(xml_data.getroot(),
                                              'policysetting')
                pv = ET.SubElement(policysetting, 'version')
                pv.text = '1'
                name = ET.SubElement(policysetting, 'name')
                name.text = 'Text File'
                description = ET.SubElement(policysetting, 'description')
                description.text = 'Represents a Generic Text File'
                apply_mode = ET.SubElement(policysetting, 'apply_mode')
                apply_mode.text = 'replace'
                data = ET.SubElement(policysetting, 'data')
                filename = ET.SubElement(data, 'filename')
                filename.text = 'motd'
            elif e.args[0] == NT_STATUS_ACCESS_DENIED:
                raise CommandError("The authenticated user does "
                                   "not have sufficient privileges")
            else:
                raise

        text = ET.SubElement(data, 'text')

When the file doesn't exist yet, it's created, and then we have "data" populated, and it can be used outside the try/except block.

If, however, the policy file exists already, no exception is raised, and we jump straight to the "text" assignment, which then fails because "data" is not defined.