This is because you're not doing anything inside of the repeat-until statement.
The code you want to be repeated until it's condition is true has to be inside of the repeat-until statement.
repeat
print(I'll be printing forever. True will never equal false.)
wait()
until true == false
The code above will print forever since true will never equal false. That was an example of how you could use it.
You could also use it like this, which will fit your needs perfectly.
db_value.Value = db
repeat
wait(1)
db_value.Value = db_value.Value -1
print(db)
until db_value.Value <= 0
print("db_value equals 0 or less.")
If you want to read more about repeat-until statements then feel free to look at the pil.
2 comments
It Lags Because You Didn't Put The Code Before The Until.
Nice answer.