go to conf/battle/skill.conf and search for this line
// Can ground skills NOT be placed underneath/near players/monsters? (Note 3)// If set, only skills with UF_NOFOOTSET set will be affected (skill_unit_db)skill_nofootset: 1
then set it to skill_nofootset: 0
then in your server type @reloadbattleconf and your done