Formula field produces no effect - blank value on field

Formula field produces no effect - blank value on field

Hi,

I am testing formula fields and I need to set-up a lead unique code.
I want to concat a static string with a substring of list field and with a string transformed auto number field.

example : PE000012
Where P is a static value,
E is the first letter of a list field,
000012 is an autonumbered field value.

Problem : when I create a new lead my formula field left blank value on detailled view of lead

here is my formula :
  1. Concat('P',Substring(${Leads.Type prospect},0,1),Tostring(${Leads.N° prospect}))