Bad value “” for attribute “itemprop” on element “X”.

The itemprop attribute needs at least one property name, so an empty value such as itemprop="" is invalid on any element.

itemprop belongs to HTML microdata. It marks an element as a property of the item created by its nearest itemscope ancestor, and its value is a space-separated list of property names, such as headline, image or author, or absolute URLs. An empty string, or a value made only of spaces, contains no names, and the validator reports it together with the element that carries it.

Empty values usually come from a theme or template that always prints the attribute and fills it in only when a property is configured. The microdata processing model skips any element whose itemprop has no names, so the attribute adds nothing to the item. On an <img>, the property value would be the image URL from src, and that value is lost.

Either write the property name your vocabulary defines (for a Schema.org Article, the main picture is image) or remove the attribute. Removing it is the right fix when the element sits outside any itemscope, because a named itemprop there triggers a different error saying the element is not a property of any item.

Invalid example

<article itemscope itemtype="https://schema.org/Article">
  <h1 itemprop="headline">Spring hiking routes</h1>
  <img src="trail.jpg" alt="A trail along a cliff" itemprop="">
</article>

Valid example

<article itemscope itemtype="https://schema.org/Article">
  <h1 itemprop="headline">Spring hiking routes</h1>
  <img src="trail.jpg" alt="A trail along a cliff" itemprop="image">
</article>

Without microdata, drop the attribute:

<article>
  <h1>Spring hiking routes</h1>
  <img src="trail.jpg" alt="A trail along a cliff">
</article>

Learn more:

MDN Reference

MDN Web Docs: itemprop

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MDN Reference

MDN Web Docs: Using microdata in HTML

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WHATWG Spec

HTML Living Standard: Microdata

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