No canonical URL
Effort to fix: Quick win (under an hour)
Why it matters
Without a canonical tag, variants of the same page (with/without www, tracking parameters) compete with each other in search.
How to fix it
Add <link rel="canonical" href="https://your-preferred-url/"> to every page, pointing at the preferred URL.
On WordPress
WordPress core adds canonicals for posts; SEO plugins add them site-wide, so check that yours is enabled for the homepage.