Humans can’t like on Neptune because we where designed to survive on Earth and Neptune does not contain our basic needs for survival.
Facts provided by google:
Neptune is very hot and is liquid like, the peak temperature of Neptune is 10,00 degrees and the human body isn’t able to survive that, you will die from a heat stroke or won’t even be able to breathe because it’s air is made of hydrogen, helium, and methane.
Hope this detained answer helped.