Please see the steps below:
1. Go to Leave > Add Adjustment > select the employee > Effective Date = Hire Date > Leave Summary = MOM Sick > Transaction Code = Entitlement Adjustment > Days = 5 > Save and Close.

2. Go to Leave > Add Adjustment > select the employee > Effective Date = Date completed 3 service months > Leave Summary = MOM Sick > Transaction Code = Entitlement Adjustment > Days = - 5 > Save and Close.

3. Go to Leave > Setup > Leave Type > click into MOM Sick > Application tab > make sure "Can apply after in service for more than: 0 Months".

Then kindly try to apply the leave again.