$field = get_field_object('domaine_de_formation');
$choices = $field['choices'];
if get_field_object(‘domaine_de_formation’) is returning false then ACF does not know what post you’re trying to get the fields from, or there are no values set for the current post. ACF cannot get the field object using the field name if values are not set for the current post or acf cannot determine what the current post is.
I will need some more context on exactly how/where/when you are attempting to use this function.
