Align item property not working

I was playing with flexbox ( actually learning it ).
My align items : stretch property not working .
I don’t know where I was going wrong .
Please help me to find my mistake
https://codepen.io/shubham10200/full/XvrgWj

Not sure what behavior your looking for but stretch is the default (default value is normal, “For flex items, the keyword behaves as stretch”).