In the screenshot below, I set a start date of 20May2016 and a lookback date of 23May2016. Note that the Start Date is at the top and the Lookback Date is one row below.
In my test, HEC-ResSim does not allow the simulation to go forward. Instead, it gives me an error message when I press "OK".
To solve this, make sure that the lookback date is at a date prior to the start date.