for 5-10 seconds is not an entity type

[Swirly]

on damage:
if damage cause is fall:
if victim is a player:
set {_damage} to damage
if {_damage} is greater than or equal to 3:
send "&cYou'd feel a sharp pain in your leg as it snapped" to victim
apply slowness of level 1 to victim for 10 seconds
apply nausea of level 1 to victim for 5 seconds

 

[senge1337]

Try this:

on damage:
    if damage cause is fall:
        if victim is a player:
            set {_damage} to damage
            if {_damage} is greater than or equal to 3:
                send "&cYou'd feel a sharp pain in your leg as it snapped" to victim
                apply potion of slowness of tier 1 to victim for 10 seconds
                apply potion of nausea of tier 1 to victim for 10 seconds