Uncaught TypeError TypeError: Cannot set properties of null (setting ‘innerHTML’) at calculateBmi at onclick

Hi guys, I am trying to create a BMI calculator. Below is my code but when I do the onclick calling a function I am getting error. However, I checked by doing an alert and the alert did workout. I think there is something error which I do not understand. Please help!!

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I did alert in function just to check and looking for a solution to surpass this error