def METHOD_NAME(addon, version_string):
    """Returns an error string if `version_string` isn't greater than the current
    approved listed version. Doesn't apply to langpacks."""
    if (
        addon
        and addon.type != amo.ADDON_LPAPP
        and (
            latest_version_string := addon.versions(manager='unfiltered_for_relations')
            .filter(channel=amo.CHANNEL_LISTED, file__is_signed=True)
            .order_by('created')
            .values_list('version', flat=True)
            .last()
        )
        and latest_version_string >= version_string
    ):
        msg = gettext(
            'Version {version_string} must be greater than the previous approved '
            'version {previous_version_string}.'
        )
        return msg.format(
            version_string=version_string,
            previous_version_string=latest_version_string,
        )