This is Flutter 2.8 bug. Change those lines in this Location:
 C:\flutter\.pub-cache\hosted\pub.dartlang.org
 \flutter_iconpicker-3.0.4\lib\IconPicker\Packs\Material.dart
// (Use double under score to the Icons.six__
 'six_ft_apart': Icons.six__ft_apart,
  'six_ft_apart_sharp': Icons.six__ft_apart_sharp,
  'six_ft_apart_rounded': Icons.six__ft_apart_rounded,
  'six_ft_apart_outlined': Icons.six__ft_apart_outlined,