Why error occurs? (null)

Question

"Cannot read property 'appendChild' of null"
why this error occurs?
please help...
https://code.sololearn.com/WVE1P7l7D933/?ref=app
https://code.sololearn.com/WVE1P7l7D933/?ref=app

Answer 1

add in the beginning of the JavaScript code and close it at the end
window.onload=function(){

Answer 2

thanks very much