What does "... name does not exist in the current context" mean?

Question

In my script, there are a lot of error called "... name does not exist in the current context" and I don't know what it means... May anyone help me, please?

https://code.sololearn.com/c4lD5mZS81Dv/?ref=app

Answer 1

Thanks a lot! But there's still an error whom don't let me use "uppercase" just once...

https://code.sololearn.com/c4lD5mZS81Dv/?ref=app