Image elements do not have [alt] attributes
Alt text for images
Alternative text for images is both the easiest and the most difficult accessibility standard. Put simply, every image MUST have Alt text - the text that screen readers and bots can read in place of the image.
Webflow automatically gives all images a null value, Alt = "", which hides the images from screen readers. So it should be impossible to fail this automatic test.
It's important that every Alt text is meaningful and you can follow the Alt Tag decision tree to see how best to write your Alt text.
Where a Webflow site fails this Lighthouse test it's because the developer has added a widget such as Facebook, Google Maps or a chat box, or the image is brought into Webflow from another server as a link. Here the image source needs to hold the Alt text.
Google Lighthouse says
Informative elements should aim for short, descriptive alternate text. Decorative elements can be ignored with an empty alt attribute.