Many people have encountered such a situation especially they opened Yahoo mail in Google Chrome. Luckily, it is not a serious issue and needs just several steps to address.
Step 1: Check the date and time of your computer. If it's not the same as online time, your browser will not verify the SSL certificate. Then it will send you a message your connection is not private. Correct your computer time and reopen Yahoo mail to see if it works.
Step 2: Clear trashy cookies and cache on your browser. You can refer to this article onhow to clear cache and cookies in different browsers.
Step 3: Your Firewall and Avti-various can cause the error. They will block the SSL certificate. Therefore, you can turn off Firewall and Avti-various to check for it.
Step 4: If the workarounds above don't work at all, try to scan your computer to see if there is a virus.