Symptoms
Unable to update a WordPress instance via Smart Update in WP Toolkit:
Smart Update test run could not be performed due to errors. Address the errors and try running Smart Update again. Malformed sitemap given
Cause
The domain's sitemap does not exist or is not valid.
The Smart Update feature uses a sitemap to check the webpages mentioned in it. If the sitemap is invalid, the Smart Update feature does not know what web pages to check so the update will not be performed.
Resolution
Contact the website developer to check the sitemap or create it if it's absent.