If your Mac is saying all websites are not secure then it could be an issue with your MAC computers clock or other settings.
Make sure the time and date is set correctly on your MAC computer.
If it's still saying not secure then it could be because the websites you're visiting have a misconfigured SSL or they have some images or files linked with http: instead of https:
By seeing the 'Not Secure” Safari message on an iPhone, iPad, or Mac you are simply being informed by Safari that the website or webpage being visited is using HTTP rather than HTTPS, or perhaps that HTTPS is misconfigured at some technical level.
By default, HTTP does not encrypt communication to and from the website.
If your website is displaying not secure it's either because of an SSL certificate issue or it could be that your images or some other files are being sent using http instead of https.
Make sure that all your images and files are linked as https: and not http:
If it's an SSL certificate issue you may need to reinstall the SSL certificate again.
The reason you are seeing the “Not Secure” warning is because the web page or website you are visiting is not providing an encrypted connection.
You can also try manually replacing HTTP with HTTPS in the URL, as some sites may have partial support for HTTPS but don't offer it by default.
Turning off not secure on Safari is easy.
To turn off not secure on Safari Web Browser follow the below instructions.
Choose Action menu > Preferences and click Security.
(The Action menu is near the upper-right corner of the Safari window, and looks like a gear.)
Deselect “Ask before sending a non-secure form to a secure website.”
When Safari says a website is not secure it could either be an issue with your Safari Web browser or computer or it could also be an issue with the SSL certificate on the website you're visiting.
First make sure the time and date on your computer is correct as that can cause an issue with the not secure with Safari web browser or other web browsers.
The browser needs the correct time to validate the SSL certificate on the website.
If that is not the problem then it may also just be an issue with the websites SSL certificate and in that case the website owner or admin would have to fix the issue.
To check if it's your browser go into the settings of your Safari Browser.
Choose Action menu > Preferences and click Security.
(The Action menu is near the upper-right corner of the Safari window, and looks like a gear.)
Deselect “Ask before sending a non-secure form to a secure website.”
1 Make Sure Date and Time are Correct. Security certificates are used by our web browsers and computers to make sure a particular site is safe.
2 Clear Browser History. After verifying that the date and time on your device is correct, test things again.
3 Try the Webpage Again.
Most web browsers alert users if they view insecure web pages by displaying a “Not Secure” warning.
This indicates the web page is not providing a secure connection to visitors.
When your browser connects to a website, it can either use the secure HTTPS or the insecure HTTP protocol.
It can also be an issue where images on the website are not being served through https and instead they are being served through http
In that case the website owner needs to fix the issue.