Catalog SEO Configuration and Store View Scope
Check the Canonical configuration for products or categories and possible theme overrides. The two side types shall not be treated the same unchecked.
Compare canonical settings for products and categories in the effective configuration range. A store view can override an inherited value; layered navigation extensions can also bring in their own rules.
Where to check the setting in Magento
Under Stores → Configuration → Catalog → Catalog → Search Engine Optimization, check the canonical specifications for products and categories in the right scope. Also, compare URL rewrites and extensions for layered navigation. A Store View deviation must not be obscured by a global setting.
Example of this specific finding
Illustrative control case: An article page and its tracking variant provide the same text, but not a preferred URL. The variants must first be compared in terms of content.
<!-- Auf Artikel und identischer Trackingvariante -->
<link rel="canonical" href="https://example.org/ratgeber/bremsen/">follow-up audit in Magento
Compare a product via direct and category path as well as a filter page. Do not force all filter combinations to the same address without checking their contents.
For these identical variants, consistently specify the clean article URL. Internal links and sitemap should give the same address.
Derogations and scope
A unique page without a canonical is not automatically unindexable. Even an existing canonical does not guarantee the selection by Google.
Reference: Magento Open Source or Adobe Commerce with classic storefront. Consider store views and extensions; other storefront architectures have their own configurations.
Classify the general error
Definition, system-independent causes and further correction examples: A missing canonical is not a blanket indexing error