As long as your new ID is at least 2 characters long, the button will turn from grey to blue so you can save it.
In your screenshot, there isn’t anything in the Set new individual ID textbox, so it’s not clear if you were unable to save a new ID even after typing something here. Let me know so I can continue troubleshooting.
In the other thread, it was a result of trying to assign the giraffe to an existing ID that belong to a different species than the one they were trying to assign. Image Analysis has returned and no match was found - #9 by Anastasia
Because you’re trying to create a new individual ID and not assign an existing one, they are two different issues.