Hi @faiza_conte! When using forms, once the form gets called for the first time, the bot will prompt the user for the next required slot value. For this you need to define responses like utter_ask_<form_name>_<slot_name> or utter_ask_<slot_name>. Please check out the rasa docs forms.
If you have more questions can you create a new question on the forum? This way other user can follow your questions more easily. 
Thanks!