Hello Dangtiep,
Thanks for your question. This error appears if tiny inaccuracy is found in your project that are smaller than the Revit tolerance. It's not easy to find. The only way is to make a little selection of some elements and THEN use the command. In that case only the formwork area of the selected elements is calculated. Do that step-by-step with all elements.
Tip: Formwork areas are calculated differently depending if neighbouring elements are selected as well.
I hope this answers your question. If you have further questions don't hesitate to answer or open a new thread in our forum.
