Join - 0 objects joined, 4 objects discarded

[a.wuppukondur]

Hi,

I want to join the lines forming each of these curved shapes. But I am not able to do so because of the error. When I zoom into the corners, the lines appear to be closed, but still I am not able to join. Please help. 

[Patchy · Accepted solution]

See your other post.

[Kent1Cooper]

They're not all in the same plane.  Getting the Surfaces out of the way, and looking from a 3D view direction:

[a.wuppukondur]

Thanks, it worked.

For reference, people facing same issue can refer to https://forums.autodesk.com/t5/autocad-forum/cannot-export-to-stl/m-p/12250699#M1115394