the websites http://www.example.com and http://example.com DOES NOT resolved to the same url

What does that mean? => the websites http://www.example.com and http://example.com DOES NOT resolved to the same url.

I have already modified from htaccess

www.example.com => https://example.com

http:// => https://

This part works normally.

Despite this modification, that message keeps coming out. I don't know what to do anymore.

What else should we do?