Grid layout and flexbox, and vanilla JavaScript

Hello Frontend community, This was a tough challenge for me, and after trying for a long time I thought it would be best to get help. I couldn't get the function to calculate as the numbers are being entered in the input fields, so I tried to create a calculate button and add an event listener to it so the function would not execute before entering the data, but then I got a type error saying "cannot read the properties of null(addEventListener)", and I can't figure out what the problem is. Any help is deeply appreciated :)