Problems with "www" and sub-pages (404 Error)

Sub-pages on my website work when there is no “www”. E.g. penthousefirm.com/pricing works.

However, when I navigate to www.penthousefirm.com/pricing, I get a 404 Not Found Error. This problem is the case for all of my website’s directories (ex: /review, /crm, /contact).

I already have a CNAME set up like this:
CNAME www penthousefirm.com

Any help would be appreciated.
Thanks