How to customize form validation error messages in Symfony?

You can customize form validation error messages in Symfony by creating a translation file for your specific form type.

Create a translation file for your form type in the translations directory of your Symfony project. The file should be named according to the locale you are using (e.g. messages.en.yaml for English).
In the translation file, specify the error messages for each validation constraint you want to customize. The keys in the file should match the validation constraint names, and the values should be the custom error messages you want to display.

For example, to customize the error message for the NotBlank constraint in a form type called MyFormType, you would add the following to your translation file:

MyFormType:
  fields:
    fieldName:
      NotBlank: "This field cannot be blank."

Make sure you have enabled translation in your Symfony project by configuring the translator service in your services.yaml file:

services:
    App\:
        resource: '../src/*'
        type:     annotation

    Symfony\Contracts\Translation\TranslatorInterface: '@translator'
    Symfony\Contracts\Translation\TranslatorInterface:
        class: 'App\Symfony\Contracts\Translation\TranslatorInterface'

Finally, set the translator default locale in your framework.yaml file and clear the cache:

framework:
    translator:
        default_path: '%kernel.project_dir%/translations'
        default_locale: en

Now, when a form is submitted with invalid data, Symfony will use the custom error messages you specified in the translation file for the corresponding validation constraints.