If a website is showing not secure on Safari web browser it could be cause the site you're visiting has not setup the SSL certificate or there are errors with the SSL certificate.
Sometimes a website may have an SSL certificate installed but it may have expired or it may be that some files such as images may be linked with http and not https
In the Safari app on your Mac, use Security preferences to turn security warnings on or off.
Also enable or disable JavaScript. To change these preferences, choose Safari > Preferences, then click Security.
Get a warning if the site you're visiting is a suspected phishing website.
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.