Copy-paste fix pattern
fn main() {
u256 coins = 250
println("Coins: " + coins)
save coins
}Why it happens
A name was read before it was declared or before a function returned it.
How to fix it
Declare the value before using it, or correct the spelling of the existing variable.
Runtime and Studio now show this kind of page from the error dialog: plain message first, stable code second, original details third.