Robin
Seems you have set up your system with a non-utf8 locale, which can and will cause lots of trouble these days.
Check:
$ locale
If something different than settings ending with .UTF-8 show up, run the User Language tool in antiX Control Centre, section System, and therein click the + button right next to the current language (screenshot 1) to select a proper UTF-8 locale (screenshot 2).
Alternatively do what the error message suggests already: reconfigure your locale on command line. If you want to do it that way, check with
$ sudo debconf-show locales
what is currently set on your system. Then:
$ sudo dpkg-reconfigure locales
(search forum, how to apply this had been discussed already several times)
In any case, make sure to set an UTF-8 locale instead of the iso88591 you are obviously using on your system for some strange reason, if the error message is correct.
Windows is like a submarine. Open a window and serious problems will start.