If you see the ‘Page not found’ error when connecting a Professional account with Content Office, then you don’t have Facebook Business Page, or it is not public.

• To check if you have Facebook Business Page, open Facebook app → Login with your Facebook account → Tap‘Pages.’ If you don’t see a Page in the list, create a new one. Read the article: How to create a Facebook Page
• Check out if your Facebook Page is public: Copy the web address of your Facebook Page → Log out of Facebook → Paste the web address into the address bar of your browser