@api.depends('category_id')
def compute_duration_display(self):
    if self.category_id == 1:
        self.number_of_days_display = 10
    elif self.category_id == 2:
        self.number_of_days_display = 10
    elif self.category_id == 3:
        self.number_of_days_display = 15
    elif self.category_id == 4:
        self.number_of_days_display = 20

i tried this code but iam getting error.