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:
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.