I got some problem with floating button :((

I have found a useful and simple tutorial for me but I cant do it,i have add the scripts in the description too. Can you guys explain for me ? Does it still work in Unity 2019.3.5f ?.(I am VietNamese sorry for bad English but plz help) .Link here:

Since you are copying from the project, Link your full script codes and the steps you made. Old unity may also cause problems, this video is old