Dear Beth Scherer,
Good day! Thank you for posting to Microsoft Community.
In order to understand your query and provide possible solution, May I please know the Outlook email client you are using? Are you using Outlook for windows, Outlook for Mac, Outlook for Mobile or Outlook on the web?
If you're using Outlook on the web or the new Outlook for Windows app, May I please know if you've checked if Show cc is selected under the options menu? If not, when you reply to an existing email, you can go to "Options" and select "Show Cc" to add the "Cc" field in the email compose window. You can refer to the screenshot below:
If you are using Outlook for Mac, in the reply email box click Cc next to "To" field. You can refer to the screenshot below:
If you are using Outlook for Mobile, after you click reply or reply all, click the dropdown arrow to add Cc field. You can refer to the screenshot below:
If your scenario is different from above, could you please provide us with the screenshot of your Outlook version details? see What version of Outlook do I have? - Microsoft Support
I look forward to your update. Please let me know how it goes.
Thank you for your precious time. Have a nice day. Sincerely,Libeamlak | Microsoft Community Moderator