The URL in review is an AMP Page URL which appears to lack a canonical tag.
Why is this important?
A canonical tag is essential for AMP pages to be valid. The canonical tag needs to link to the original non-AMP version of your page, or be self-referential if the page is a standalone AMP page.
Without a canonical tag, search engines might not deem the page as valid HTML and could possibly exclude it from search results or not index it correctly.
What does the Optimization check?
The Optimization will be activated for any AMP Page URL that's devoid of a canonical link in both the HTML markup and the HTTP header.
How do you resolve this issue?
Ensure that the canonical tag is placed on the AMP page to point to the regular version of the page, and have the non-AMP page refer back to itself with a canonical tag.
Here's an example setup:
Given the URL: https://example.com/page-c/
This is how you define a self-referencing canonical and an AMP page:
The respective AMP page would have a canonical pointing to the non-AMP version:
<link rel="canonical" href="https://example.com/page-c/" />
For a standalone AMP page, e.g., https://example.com/page-d/amp/, the canonical should reference itself:
<link rel="canonical" href="https://example.com/page-d/amp/" />