Adding a Holiday on behalf of staff

Hi our managers are currently adding staff holidays. One employee in has a pro-rata allocation of 15 days, the manager wanted to add 8 paid days holiday over a period of 18 days but mark the rest as unpaid so that the person could not be added to the rota as she is on holiday. The manager was not allowed to do this and got an error message, is this because the original overall days requested was higher than the allocation ? Should it be entered in smaller chunks ?