1st case: the 13th is not calculated.

Check that the right to the 13th is indicated in the management of the employees in the tab 5 (Salary data - 13th salary):

If the fields are empty, it will not be calculated automatically. Fill in the correct information then redo the total 13th.

2nd case : the amount is wrong.

It is possible that the entitlement to the 13th has been filled in later and that some of it is missing. In this case, redo the total13th.