Hello !
To use Copilot in Outlook, make sure you have a Microsoft 365 Personal / Family subscription. You can check this at https://account.microsoft.com/services.
Check that your Office apps are up-to-date, and enable connected experiences by going into File > Account > Account Privacy > Manage settings, and enable "Experiences that analyze your content".
You can find the complete tutorial on Microsoft's support center here : https://support.microsoft.com/en-us/office/how-to-find-and-enable-missing-copilot-button-in-microsoft-365-apps-c8482b93-4b96-4bb8-8ec9-5148f4d42441
Please let me know if you have any other question !