Quick answer: Each indexable URL needs a self-referencing canonical that matches the live address, including www and slash.
Make every indexable page point at itself. Redirect the rest. This guide is written for businesses in the UK and Kent, Surrey and Greater London.
Self-referencing is the default
The canonical should equal the URL in the address bar after redirects.
Why GSC complains
Alternative page with proper canonical means Google found a duplicate and believed your tag. Fix the duplicate with a 301.
Slash policy
Pick one. Redirect the other. Do not 200 both.
Implementation
SEO Friendly Website Design sets this in .htaccess and the HTML head. Ask for a canonical pass.
Frequently asked questions
Can I canonical to a different domain?
Only for true copies you do not want indexed.
Does canonical replace a redirect?
No. Redirect users; canonical is a hint for copies.
What about parameters?
Strip or noindex them. Do not let them rank.