robot.txt issue

Hi there,

When I type my URL with “WWW”: https://www.mywebsite.com/robots.txt
it shows a message below:
“User-agent: *
Disallow: /
<!– WP Super Cache is installed but broken. The constant WPCACHEHOME must be set in the file wp-config.php and point at the WP Super Cache plugin directory. –>”

So I didn’t see my website showing at Google. After I modified the robot.txt with a plugin called yoast. I can see the new robot at URL:
https://mywebsite.com/robots.txt (WITHOUT WWW)

My question is how to remove/modify the robots at the link with WWW?
I have modify .htaccess file and redirect plugin. None of them is working.

Please advice!

Thanks