为什么我不能使用xx [this.index] .style.display？它说“无法读取undefinedbtn的属性'风格”

i want to make xx[this.index] in a loop for making each one has an index and then when i click a button of them, the background of button would turn to yellow color & the div block would change the words at the same time. why it doesn't work? i saw other people use this way is working good. how can I do? thanks a lot!i want to make xx[this.index] in a loop for mak