Fix

employee.py

@@ -31,7 +31,8 @@ class Employee(metaclass=PoolMeta):
             else:
                 return self.contract.end_date or None
 
-    def create_mandatory_wages(self, employees_ids):
+    @classmethod
+    def create_mandatory_wages(cls, employees_ids):
         MandatoryWage = Pool().get('staff.payroll.mandatory_wage')
         employees = Employee.browse(employees_ids)
         values = []