E3006

There is no individual visibility control for enum constructors.

Usually, this means that you put an priv or pub visibility modifier on a enum constructor.

Erroneous example

enum A {
  priv A1 // Error: No individual visibility for enum constructor.
  pub A2 // Error: No individual visibility for enum constructor.
}

Suggestion

Remove the visibility modifier from the definition of the enum constructor:

enum A {
  A1
  A2
}