Trying to make...a script that....?

Only allows players to take damage from a sword only when they are standing on a certain platform, just like the way it is in the game "Fencing"

And how would I set it up?

I would have it so where you step on a platform a boolvalue is set to true indicating you stepped on the platform and damage will be taken. As well as a disconnection method to make the boolvalue false.

then have the sword check to see if the boolvalue is true before dealing damage.