This issue normally occurs when there is already an existing leave request within the date range of the timesheet.
If a leave request for the period is already in place, then when you open the timesheet, you will see the leave that has been applied for.
If there are any additional days available, you will be able to use those to enter the normal hours. But if the leave is for the whole timesheet period, then you will not be able to add any further hours to the timesheet or submit it.