You can. The Problem is that you created a class. To access anything that is in the class you must create an object of that class and access the elemets than. Think of it as an hotel. If you just say room no. 1, the compiler does not know to which hotel it has to go in order to access room no. 1.
You can avoid this if you just don't use classes at all. just declare your functions as you would do in a sketch.
If you want to use classes, than you have to access the function through an object:
Uart_WIFI object(1);
object.Gestion_de_donnee(...);
The dot-operator is telling the compiler use the fuction from this class.
Just as a design hint. Do not use an initialize function. Initialize all the things in your constructor. Otherwise a user might forget to call initialize and the code does not run as expected.