If you want to buy Terraformed Islands or Summer Race, the required amount is in blue currency. This amount has to be saved in your water park. 



If you are trying to buy Desert Island, please check the following measure list: 

Millions
Billions
Trillions
quadrillions
Quintillions
sextillions
Septillions
Octillions
Nonillions
decillions
Undecillions
Duodecillions

As you see, some of them start with the same letter.

If the game is asking for Quintillions in order to unlock a new upgrade, check your current amount of money, because it it possible you have quadrillions.