In google search console, it reports my URL is duplicate and excluded because the submitted URL is not selected as cannonical.

In the inspection result page, it sasys:

User-declared canonical: https://a.b.c/index.html

Google-selected canonical: https://a.b.c/index.html

It is noticed that the only difference is the scheme. For URL submmited, it is "https", but for selected by Google, it is "http".

In fact, both the above URLs are one same page. I have put the canonical tag (href="https://a.b.c/index.html") in the page header meta.

Why does Google still select http as cannonical, regardless my assignment? How can I fix it?

Thank you very much!