Workflow rules between two modules

Workflow rules between two modules

Hello everyone,

I have a problem, I want to automatically update the tasks to close them, when a lead is closed.
It looks simple, even I'm still a novice.

So first, I created an update field for the task to change the status to closed.
Second, I created a rule for the lead we lost.

But I don't success to link them.
In the workflow rules, when I have to choose from the selection list, I can not find the "update field" associated with the task or any other update field related to task.

Do you know why? Thanks for your help !