Problem with user routine (USERELEM) of Ansys

Dear imechanica friends,

I am using userelem in Ansys. When I model a curved shaped structure like a shell, and mesh it in the context of the element ET300, I get error below:

‘unable to mesh area 3 because of shape problems. It may help to change element size on edges of area’

When I use a trick to call ET181 and then mesh, and then change the element type to ET300, the following error is appeared!

‘Negative element Jacobian value 0 at element 10. This is due to wrong element order or bad mesh’

My USER300 code works for plate, the problem only happens when I model a curved surface! Does anybody have an opinion?

Best!