The non-working days are determined by:
The Work Pattern: Weekends are non-working days.
Public Holidays: All Public Holidays are non-working days.
You are not able to book holidays on non-working days.
📌Note: Even if the Public Holiday's duration is half a day, you won't be able to book the other half as Public Holidays are non working days.
This can also be a glitch in the system. If the above doesn't apply, refresh their planner by assigning them to a no work pattern or another one using the same effective date and then assign the user to the correct work pattern and try again.