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



If you have enough money to buy a ride but the button is still greyed out, please check the requirements in red next to the BUY button:


Here is the money 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 is it possible you have quadrillions.