u is not defined???

Question

https://code.sololearn.com/WoOCdvsepc7G/?ref=app
Again,I'm a beginner at website development,(why I need to say that)
BTW,it said that function u is not defined on the html line 9 (or maybe js,idk)

Answer 1

Chang your: function u(){}
Into:
document.querySelector("button").onclick=()=>{}