Missing canonical
What it means
These indexable pages declare no canonical URL. It is not required, but without one any URL variant that reaches the page (a tracking parameter, a trailing slash, an uppercase path) can be indexed as a separate page.
How to fix it
Add a <link rel="canonical" href="…"> that points at the page itself, with the page's absolute, preferred URL. Most CMSs and frameworks can do this for every page from one template change.
Finding it in CrawlStat
After a crawl, open Issues and pick Missing canonical. Click Show all in Pages to filter the Pages table to the affected pages, or Export affected to hand the list to whoever owns the fix. Audit ID: canonical_missing.