Good Day,

Does the leave definition allow the employees to accumulate for leave days during this period ?

If not then you will have to adjust the employee's balance then the employee will be able to apply for leave or 

you can amend your leave definition to allow employees to accumulate leave during the probation period.

To adjust employee's leave balance 

From Navigation pane

Select  Employee 

Access Leave Detail

Select Leave definition 

on the leave definition type in the adjustment value.