If you see the error message 「Undefined」 when you try to add a template:
Upgrade your PHP version to PHP7 (backup your website before)In some cases, the latest version of PHP may cause undefined errors. Although staying up to date with PHP versions is often a good idea, it may cause problems for edge cases. Downgrade to a PHP version one version lower than the latest if this occurs.Ask your hosting company to increase the timeout value of your serverAsk them also to check the database encodingVerify that it is not due to a plugin conflict by deactivating all your plugins besides Elementor and Elementor Pro. If this solves the issue, reactivate them one by one to find the culprit.Check if it is not due to a conflict with a browser add-on by editing in incognito mode with your browser.