The warning "Low text to HTML ratio" may happen on your AMP pages if you have considerably short collection or product descriptions, or if you have a lot of products per collection page.  All this warning means is that the page has what SEMrush deems "not enough text" in comparison with the amount of HTML.

On Collection Pages

The solution is to either:

• Lower the number of products per collection page,
• Increase your collection description to be much longer

When this SEMrush warning arises, it arises most typically because you have more than 8 products on a collection page, and you have a considerably short collection description.

As you adjust your theme to have more products visible on a collection page, this increases the HTML needed to render the page.  In SEMrush's eyes, too much of this can cause "Low text to HTML ratio".

You can "fix" this if you want, by the recommendations above, but keep in mind that collection pages aren't meant to have a lot of text, they're meant to have images, and Google knows this.  It might be in your best interest to simply ignore this warning.  

On Product Pages

The solution is to increase your Product Description.  You can do this within your Shopify Admin Panel.

Is this warning relevant?

Please also note that this warning from SEMrush is just a warning -- it doesn't mean Google will think the same thing that SEMrush thinks.  Google is quite smart and knows when a page is not displaying proper content.  A page with big images might be perfectly fine when there is little text.  We recommend taking everything that SEMrush "warns" about, with a grain of salt.