Because every version of flutter_localizations from sdk depends on intl 0.17.0 and fstore depends on intl ^0.16.1 code example

Example 1: Because every version of flutter_localizations from sdk depends

flutter channel stable
flutter upgrade

Example 2: Because every version of flutter_localizations from sdk depends on

Adding this to pubspec.yaml solved it for me:

dependency_overrides:
  intl: 0.17.0-nullsafety.2
  
This is versioning issue. I would advice you to update your Flutter SDK 
(from flutter_localizations from sdk depends on intl 0.16.0 is visible 
that you are using some of the old versions, because last versions are
compatible with intl 0.16.1. If you don't want to update it, you can use 
another lower version of flutter_form_builder < 3.7.3 which will be compatible
with intl 0.16.0