DAMBRK will warn you that it is needing to extend the cross sectional geometry upwards to contain your routed flow (if the analysis makes it that far).
I would recommend "simplifying" the model down, so you can determine the source of the error. I sometimes will delete cross sections, one by one, off the downstream end of the model until I can get DAMBRK to run. This allows me to determine which cross section(s) are causing problems.
I would not recommend adding floodplain top width areas. In my experience, it only compounds the problem.
Sincerely,
Chris E. Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com