The site is not reachable over https

What this means

No https address for this origin answered with a status between 200 and 399, while an http address returned 200. Every visitor is served over an unencrypted connection that browsers label as not secure, anything they submit is interceptable, and HTTPS has been a Google ranking signal since 2014. This is the one host check permitted to read an unreachable address as a defect, so it is guarded twice: an http variant must have succeeded before it can fire, which means a network failure affecting every probe produces no finding at all.

How to fix it

Install a TLS certificate — Let's Encrypt issues them free and renews them automatically — serve the site on port 443, then 301-redirect all http traffic to https and enable HSTS.

Fix effort

This check needs a developer fix: it turns on a routing, hosting or infrastructure decision that no automatic edit can make safely.

Authoritative source

https://developers.google.com/search/docs/crawling-indexing/https