Please correct my script on this code, keeps giving me an error (line 16)

Question

https://code.sololearn.com/W96zpjNeNASM/?ref=app

Answer 1

Dom is not loaded.
put line 15 ff into eventhandler window.load

Answer 2

https://code.sololearn.com/WWxRtV2V9jQG/?ref=app

look line 19 js part

Answer 3

can you please show me how to do it? I'm still learning

Answer 4

Thank you very much, makes more sense now.