not working when I don’t enter www before domain name

I have WordPress embedded into a website layout and when you enter the domain name with the www before it everything works fine. But when you enter the domain name without the www there is an error.

Error is:
Warning: Cannot modify header information – headers already sent by (output started at /home/content/r/e/p/replaysports/html/dev/blog/index.php:8) in /home/content/r/e/p/replaysports/html/dev/blog/wp-includes/pluggable.php on line 770

Please check out https://replaysportsmonthly.com/dev/blog/index.php to see error on the page.

Any ideas? Thanks!