Error Log: you can see in the log that error is coming 3 times because I am adding Employee object 3 times in ManageEmployee.java class with addEmployee(...) method so I am assuming that problem is with the sequence generator for the column id.
Here I am using naive generator. i.g. Integer empID1 = ME.addEmployee("Zara", "Ali", 1000);
Have you asked Hibernate to create your db for you, or have you done it by hand?
If the latter then you will (IIRC) need to create the SEQUENCE that Hibernate is to use for your IDs, which will also mean changing your XML to tell Hibernate what SEQUENCE to use.