404 Not Found HTTP code return is incorrectly configured

Yandex writes:
Probably, the site https://board.rusknr.ru incorrectly configured to return the HTTP code 404 Not Found, which can adversely affect the indexing of the site by the robot. Configure the 404 code to be returned to request pages that do not exist.
The site is on Joomla. You need to set it up. Interesting low cost.