The error appears although I have the same script as in minute 4
The error says there's ambiguity? If so then more than likely it does not like how things are named. Everything written in the video is correct. I don't receive any errors when writing it out that way. Make sure when naming variables that you avoid using the exact same name as the type.
I would rename that variable to something a bit different like "ball" and try it again. Also make sure the error is coming from the exact script and not another script.