I had the same problem.
According to this article https://support.microsoft.com/en-us/office/draft-an-email-message-with-copilot-in-outlook-3eb1d053-89b8-491c-8a6e-746015238d9b "Drafting (with Copilot) in Outlook for Microsoft 365 is currently available in new Outlook for Windows, Outlook on the web, and Outlook.com. If you have classic Outlook, Get new Outlook."
The problem is that the "New Outlook" is considerably limited in functionality especially for elaborate setups with multiple accounts, many inbox rules, etc. Like another person commented, it is either "Classic Outlook with all its features" or "New Outlook with limited features and Copilot".
Typical Microsoft bullsh*t in my opinion. I am cancelling my Copilot trial.