Slider javascript

Question

Can anyone explain, why code doesn't work and why everytime I run the code comes up this error:
JavaScrypt
Line: 6Uncaught TypeError: Cannot set property 'className' of undefined
Line: 26Uncaught TypeError: Cannot set property 'className' of undefined
https://code.sololearn.com/WyWDUKV2ae2A/#html

Answer 1

https://www.sololearn.com/post/7444/?ref=app