How to creat 2D element from nodes

hi, everyone
i'm a beginner in Hypermesh. Recently, i've encountered a problem in 2D meshing. I  have 3 nodes (like in the photo below). I want to create a trias element from them, or creat some lines from them, so i go to Mesh - create - 2D element - Element. I chose those 3 nodes, but no element was created!!!
So could anyone tell me where i was wrong and how to fix this, thank you very much!!

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