Solved.

I was declaring in the wrong way the method ' process_demo_scheduler_queue '; i forgot that in python, methods and variables begin after four(4) white spaces before declaring them.

Right way:

class scheduler_demo(models.Model):

     _name = "scheduler.demo"

     # fields ...

     #declare de method, but first give 4 white space

     # method declared with 4 white spaces

     def process_demo_scheduler_queue

          #put here your custom code

wrong way:

class scheduler_demo(models.Model):

     _name = "scheduler.demo"

     # fields ...

     #declare de method, but first give 4 white space

     # method declared with no 4 white spaces

def process_demo_scheduler_queue

    #put here your custom code