In order to use your Plesk server to request a LetsEncrypt certificate for a domain, you need to first set up the domain on the same Plesk server and test it to be sure the site (ie http://giftsplendid.fun) loads the default Plesk page or your custom website if you've uploaded it.
But when I load http://giftsplendid.fun, it opens the default Nginx page, not the default Plesk page or any custom website.
Which makes me suspect the domain may not have been pointed to the same Plesk server you're using to request the LetsEncrypt certificate, hence the error message you're seeing.