Answer 1

Your Mom Read the message error. It tells a lot.
Then read about getElementById method.
You're trying to use in JS a reference only present in HTML.

Answer 2

Please don't delete the code. When you do so then all the comments posted become useless.

Answer 3

In general, the code is too confusing and repetitive, making it difficult to debug. Some questions:

1. Line 4 seems invalid. What is its purpose?
let time = document.getElementById("timer").innerHTML = timer--;
2. There are 5 identical functions for the boxes. Why are them separated, instead of a single one with box as a parameter?
3. How is the code supposed to behave?

My advice is to take a step back and think the logic. Maybe it just got too far before being well structured, and this makes it hard for you.

Answer 4

Your Mom I still think a step back would be the best now. I'm struggling a lot to follow the logic, and can't figure what the code should do.
Can you explain how this should work?