Based on that message, I gather you're using the Rational method with a Tc less than 5 minutes?
Most IDF curves don't define durations that small, so most stormwater regulations generally define a minimum Tc of 5 minutes or so. You can either increase the Tc manually, or you can set a minimum value that is automatically applied to each subcatchment. You will find this on the Advanced tab of the Settings|Calculation screen.
If for some reason you REALLY need a smaller time-step, the limits and decimal places can be customized on the Settings|Units screen. But I doubt this is required, and could raise other issues.