Unity3D: Why can't I select a script?

Question

Game development

There is an ax model, I create a basic C # script, hang it on the ax, the component appears, but I can’t select another script. (I will need to paste another script there later).
Here is a screenshot:

Here is the code:

Answer 1

In some of the 5 versions of the unit, this field has become immutable. And the video tutorial is probably a bit outdated.

Answer 2

Show the code of your Item.