After the page loads, hit ctrl + shift + i to open the developer tools. On the main bar, click "security" (may have to click >> first to get to it).

Once on the security page, hit F5 to refresh the page.

Then, look at the Security overview to see if it explains why. Also, in the left-hand pane, see if there are any non-secure origins listed, and click on them. Maybe it'll tell you why.

Things that could be causing the issue:

You have some extension installed that's inserting insecure content on the page. Goto the URL opera:/extensions, disable an extension and try things again. Repeat until you find the problem extension.

Your time, date, time zone and or daylight saving time settings could be incorrect. Make sure they're all good.

In Internet Properties in Windows, you could have a proxy set that causes Opera to run through the proxy. The proxy could be inserting insecure content. This usually happens if you have malware, which you should check for with Malwarebytes for example. If this is the case, it will affect Microsoft Edge too and any other Chromium browser (but not Firefox if it's set to "no proxy" in its settings).

You can also test in a private window in Opera. If it works there, it probably is one of your extensions. Or, you might have to clear all your cache and cookies for the problem sites. Or, when on a problem site, hit ctrl + shift + r or ctrl + F5 to see if that fixes it.

Also, at the URL opera://settings/userInterface in Opera, you can set "show full URL" to easily see if you're on an http or https page.

I have not been able to access specific websites like Wikipedia and Quora on my Windows. How do I fix Your Connection is Not Private error in Opera Browser to access Wikipedia and Quora

Isn't that normal?