Hello Doan,
Have you checked the processed data for the cross-section located at FLOODPLAIN 2: 0.000 meter ? Also, there are some tips about how to set up the link channel. Can you provide more details about your project (purpose of the model, relevant print-screens from your Mike setup, why you was forced to made up the floodplain geometry or any other information to clarify the context and solve the error) ?
Kind regards,
Aurelian