The problem could simply be that you haven't specified a structure ID.
When working with a dambreak structure, the ID is mandatory, and must be specified both in the *.nwk11 file, and in the boundary condition file. If you haven't specified one of them, or if they don't match, then you'll get this error.
The aim of this ID is to allow multiple dambreaks at the same location: therefore you're allowed to have multiple dambreaks on the same branch and chainage, and the ID is used to identify to which dambreak the boundary conditions apply to.
Best regards,
Mathieu