Can someone kindly explain what 'Crawl Issue Found: No rel="canonical" Tags' means? Is this a critical error and how can it be rectified?

[JoshMcLean]

Can someone kindly explain what 'Crawl Issue Found: No rel="canonical" Tags' means? Is this a critical error and how can it be rectified?

[KevinBudzynski]

It means that you are not using that tag on that page. Do you have duplicate content? If so, try to clean it up by creating unique content or use proper tags to mark duplicates. For further reading, see: http://moz.com/blog/complete-guide-to-rel-canonical-how-to-and-why-not & https://support.google.com/webmasters/answer/66359?hl=en