TypeError: 'float' object is not subscriptable

PizzaChange=float(input("What would you like the new price for all standard pizzas to be? "))      
for i,price in enumerate(PriceList):
  PriceList[i] = PizzaChange + 3*int(i>=7)

PriceList[0] is a float. PriceList[0][1] is trying to access the first element of a float. Instead, do

PriceList[0] = PriceList[1] = ...code omitted... = PriceList[6] = PizzaChange

or

PriceList[0:7] = [PizzaChange]*7

PriceList[0][1][2][3][4][5][6]

This says: go to the 1st item of my collection PriceList. That thing is a collection; get its 2nd item. That thing is a collection; get its 3rd...

Instead, you want slicing:

PriceList[:7] = [PizzaChange]*7