honest vessel
Description: If you have bought all DLCs and unlocked all characters from the base game and all DLCs, then the final secret character of the Ode to Castlevania, ||Chaos||, is unpurchasable. It costs over 17 million gold to unlock and the maximum gold you can have is one less than 10 million.
OS/Platform: Android 14
Version: See screenshot
acoustic verge
It's just a visual limit, you can have much more gold than that.
It's hard to tell how much gold you actually have once past that limit though, so all you can do is farm more gold and keep checking until you can buy it.