This issue usually occurs because the balance hasn't been configured to include this employee status i.e. PP, PW, ST etc. To fix this, follow the steps below.
Portal
Go to People Management/ WFM Admin Dashboard.
If on WFM Admin Dashboard, select WFM Person Maintenance.
If on People Management, click on 'View' on the Employees section.
Query the employee.
Click on Balances.
Select Entitlement Basis.
Check and note down the Employee Status.
Back-Office
Go to CoreTime.
Click on System Params.
Select Balances.
Select Pro Rata Entitlements.
If the Balance Code DOES NOT have the Employee Status, then add a new row including Balance Code and Employee Status.
Set the Pro Rata Leave Date option to Yes.
Click Save.
Go to People Management/ WFM Admin Dashboard.
If on WFM Admin Dashboard, select WFM Person Maintenance.
If on People Management, click on 'View' on the Employees section.
Query the employee.
Click on Balances.
Select Recalculate.