Why, when dragging a script onto an object, does unity write that the script has not been added?

Question

C++ / C#

Why, when dragging a script onto an object, does unity write that the script has not been added? The story is such that when I make the scene for the first time, everything works fine, but if I want to reopen it or in another version, unity says that I need to finish the scene, or something like that; By the way, if you create the scene first, then everything works, the script is in C #, I read in the comments that some have the same problem; what do you advise?
The script is complete.

Answer 1

Can you show an example of a fully completed script?