URL Vulnerability: Configured wrong

Testing my pages for SEO
(using https://sitechecker.pro/seo-report)
I always get this message saying there is an error on my page:

URL Vulnerability: Configured wrong
Search engines see your https://www.112ink.nl and https://www.112ink.nl/index.html (or https://www.112ink.nl/index.php) as different pages.
If you don't explicitly tell Google which URL is canonical, Google will make the choice for you, or might consider them both of equal weight, which might lead to unwanted behavior.

Does anyone know how to fix this.

There are countless SEO links optimization extensions available on the Marketplace, even free ones, that could provide better accuracy for the URL structures compared to what the core provides already. I would suggest to search for them on the Marketplace.