Hey Carolyn,
Rich text truncation (i.e. the “show more” / “show less” functionality is only supported starting with Theme 1.29. You can read more about it in this Community post or this training. You’ll have to upgrade your experience if you want to use this feature!
As mentioned in the training, for Theme 1.28 and below, you’ll need to disable truncation completely since it’s not supported - you’ll risk cutting off a closing HTML tag and breaking the page.