Hi @Annie Smith ,
Welcome to our forum!
Looks like your issue is more related to add-ins development, please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client. Based on my research, i found that there are some limitations on developing add-ins in New Outlook, please refer to:
Add-ins in the new Outlook for Windows
Develop Outlook add-ins for the new Outlook on Windows (preview)
Hope the above help! In order to better solve your problem, we will add "Office Development" tag. Thanks for your understanding and support!
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.