404 page error

I need some technical help for a website that I put together. I am unable to pull up my website whenever I use www. in front of the address. I understand this is a WordPress blog feature, but with this being a professional website that is listed on business cards, promotional materials, etc.; using the www. needs to work as well as not using it.

The website I am referencing is rpui.com

I have exhausted my search for the past 3 days and have tried everything possible to get this to work. From changing .htaccess files to 404 page redirect to changing the address on the general settings page.

I am not the best at coding, and what I have learned has been through trial and error.

Any help on this matter would be greatly appreciated!