How are you putting in your initial drawdown?
In RORB, there are 2 ways of specifying drawdown, one is to specify it in volume (m³) and the other is to input it as water level. If you put it in as volume it needs to be a negative number. Water level is a positive number.
I think you are putting it in as drawdown in metres which RORB won’t deal with correctly. If you are putting in “1” RORB will assume that it is a starting water level of 1m not 1m drawdown. This will give an error as your H-S relationship is not defined at 1m (it starts at 949.5m)
Let me know if this helps.