I have installed SSL by below method:

For SSL : Go to cPanel > Security > Let's Encrypt SSL > Issue

(But before doing this make sure you have pointed your domain to our nameservers)

But still its giving SSL error on my site, why?

After installing SSL you may need to redirect your http traffic to https.

Redirecting http to https may fix your issue.

Here is how you can do it: https://www.freecodecamp.org/news/how-to-redirect-http-to-https-using-htaccess/