It appears that your dock has been locked somehow. There won't be a way to disable it through Settings, instead the Terminal needs to be used, therefore go to the search on the upper-right and type Terminal. Once it's opened, type
defaults write com.apple.Dock size-immutable -bool no; killall Dock. This will kill the down, apply the setting, and restart it. After this procedure, drag and drop the icon to trash and Empty the Trash and that's it.