The next validation error message is interesting:
The tag 'img' may only appear as a descendant of tag 'noscript'. Did you mean 'amp-img'?
This error deserves special attention: it's the first error that's complaining about what looks to be standard use of an HTML tag. It's basically telling us that the HTML <img> tag is not permitted in AMP, and it's suggesting an alternative <amp-img>.
Let's try to swap in <amp-img> in place of our <img> tag. Copy this line in instead:
<amp-img src="https://theampbook.com/ch2/lightning.jpg"></amp-img>
Note the closing </amp-img> tag. The HTML img tag is a void tag, which means that it doesn't need to be explicitly closed. However, the same does not hold for amp-img; so you must close it explicitly...
