There are two options:
We can specify that it is mandatory to fill in the comments field. In that case, unavailability cannot be saved without completion.
We can add a number of shift types to the unavailability, for example school/holiday/sports etc. We can make it obligatory for an employee to select either of these shift types.
Attention: When you wish to implement one of these options, you have to request this via email@example.com.