I can not understand the reason. Gives an error message. What's wrong?

Question

When I select the size, I get an error, I can not understand why.
https://code.sololearn.com/Wku29nGV13pc/?ref=app

Answer 1

You never made an element with the ID showWidth

Answer 2

<div>
<button onclick='clearCanvas()'>CLEAR</button>
<button id='ERASER'
onclick='changeColor("white")'
ontouchstart='changeColor("while")'
onclick='clearCanvas()'>ERASER</button>
<div>Width:
<span id="showWidth">40</div>
</span></div>