Why is sketchware showing that name xxx is not available

There are quite a few answers, actually. It's mostly because there is another variable with the same name. But for better debugging of this project, we first need to know how what's variable. Even if it sounds trivial, it isn't.

What's variable

A variable can be multiple things:

• Literal variable. These are created in logic editor by clicking Ads variable in variable tab.
• Widget. A widget id must be unique. The ID is stored as a variable, thus the widget can be a variable. That's also why you can see widgets id inside variable tab in code editor.
• Component. As we have pointed out in Create your first project, the components are actually a classes which are saved in a variable.

This brings us to a new question:

Why we can't have duplicate variable names

This seems quite obvious (and it realy is). You can't have variable a = "dummy string" and use a object named a. This would lead into problems like a.setText() is not function. Also another reason could be how Android handles types. Learn what a type is here. You can't assign a string to number variable. That's like trying to store water in a bag. (Don't try it). Although this type juggling (You can learn more at Google) is possible in certain languages (php, Javascript etc.), it is not in Android (as it's more bug-safe and has certain performance advantages.)