It looks like you are trying to set elements 0 through 11 of PriceList to new values. The syntax would usually look like this:
prompt = "What would you like the new price for all standard pizzas to be? "
PizzaChange = float(input(prompt))
for i in [0, 1, 2, 3, 4, 5, 6]: PriceList[i] = PizzaChange
for i in [7, 8, 9, 10, 11]: PriceList[i] = PizzaChange + 3
If they are always consecutive ranges, then it's even simpler to write:
prompt = "What would you like the new price for all standard pizzas to be? "
PizzaChange = float(input(prompt))
for i in range(0, 7): PriceList[i] = PizzaChange
for i in range(7, 12): PriceList[i] = PizzaChange + 3
For reference, PriceList[0][1][2][3][4][5][6] refers to "Element 6 of element 5 of element 4 of element 3 of element 2 of element 1 of element 0 of PriceList. Put another way, it's the same as ((((((PriceList[0])[1])[2])[3])[4])[5])[6].