Humanforce has the ability to restrict Leave or Availability being requested within X number of days of the date being requested.
This is an Access Level based restriction, so it can be different for each Access Level.
To View or Edit this setting, go to Admin > Access Level. Select the Access Level you wish to view. On the General tab you will see the setting Availability change min. days notice.
You can adjust this accordingly to suit your requirements. If you wish to disable this setting simply place in a zero.