I have created a shape and selected a layer.
In the Edit menu
Define custom shape... is disabled.
Can any one tell what mistake I have made.
Select the path on the canvas with the Direct Selection Tool (white arrow). Define Custom Shape should activate for you then.
You have too many shapes delete some it will get activated.
Did this happen to me recently, it took me a long time to figure out.
You need to create a "Shape layer" or "Work path" to be able to define a custom shape.
1) As first click on rectangle tool and then on shape layers selector
2) After that draw a new layer and click with a right mouse button on that layer. You will see "Define Custom shape" option
3) Or you can create a new layer with a pen tool
4) And then take a "Path selection tool" and right-click this layer