Go to “Administration – Settings – Absence types”.

If ‘allow negative’ is checked and/or ‘balance management’ is on, you will not be able to select this type of absence when booking in an absence.

If “Balance management” is ticked/enabled, the staff member needs balance available.
You can find out how to do that here.

If the staff member does not need balance available, for example for illness/short-term leave, adjust the absence type. You can do this via the blue pencil icon next to the type of absence. Find the instructions for this here.

Tick the “Allow negative” box.