We know from your other thread that:
this is a brand new 38 litre/10 gallon tank, set up about 9 days ago and the fish added a day later.
it was not cycled before fish were put in the tank, so this is fish-in cycle
we don't have readings for ammonia and nitrite
The main problem is that the tank was not cycled before getting fish so you are now doing a fish-in cycle. Unless you've been doing water changes of at least 50% every day, ammonia will have been building up in the tank and will poison the fish. The one which has died was probably the weakest and succumbed to the ammonia faster than the others.
In order to save the rest of the fish you need to start doing daily water changes. Buy a test kit and once you have it test for ammonia and nitrite every day. Do a water change every time you see ammonia or nitrite higher than zero. Add some live plants, floating ones are best - plants take up ammonia as fertiliser and will help to reduce the ammonia level in the tank. Get some tetra Safe Start - this is one of the better bottled bacteria products and will speed up the cycle.
Finally, if at all possible get a bigger tank. Platies need a tank at least 24 inches/60 cm long. If you do this, move the filter over to the new tank at the same time as the fish, or put all the current filter media into the new filter.