Why does "Save" button doesn't save the current state of the task form?

Hi folks,

I have a task form like the following one in screenshot:

When I click on Save button so as to save the current state of my form to complete it later, this button is disabled for some reason.
And when I switch between another tab and coming back in Form tab, nothing is saved (as you can see here).

Does anyone know please what must I do in order to make it play ?

Thank you in advance,
Steve