how to get rid of this error?
Hi insafetdin,
Welcome to the community!
T-Spline bodies will be automatically converted to BRep bodies when you finish form editing. If T-Spline faces or edges cross over each other, failures would occur. It’s a good idea to use the Utilities->Display Mode to review the model in box mode because intersections are more easily discovered in box mode.
You may use Edit Form to adjust edge or vertices to correct the problems. Here is a video to show the workflow:
http://fusion360.autodesk.com/learning/learning.html?guid=GUID-D1A24FEE-1E67-4254-ACBC-EEECBE1CB97D
Best Regards,
Nicolas Xu
Sr. SQA Eng.
Fusion 360 Quality Assurance Team
Autodesk, Inc.