dconf change a string key

The value needs additional quoting i.e. to assign GVariant string value 'foo' you need to write the value argument as "'foo'"

dconf write /org/gnome/nautilus/preferences/show-directory-item-counts "'never'"

See dconf — Simple tool for manipulating a dconf database at https://developer.gnome.org/

What steeldriver said is the right way to do it. However using gsettings is an easier way to archive the same.

gsettings set org.gnome.nautilus.preferences show-directory-item-counts never