Outlook add-in works in Outlook Web but not in Desktop app

Dear Poonia, Dev ManisManish,  

Thank you for posting your question in the Microsoft Q&A forum. 

As forum moderators, we do not have access to the detailed system information required to troubleshoot this matter effectively due to privacy and security constraints. For precise assistance, we strongly recommend contacting Microsoft’s support team directly, as they are best equipped to investigate and resolve such technical issues efficiently.

For the standard procedure outlined above, Office 365 global administrators may need to contact the Office 365 support team by submitting a service request.    

Here’s what your Office 365 Global Admin should do next:          

1. Submit a Service Request: The Global Admin should create a service request with Microsoft support to get advanced technical assistance and diagnostics. For detailed instructions on how to get support, please refer to Get support - Microsoft 365 admin | Microsoft Learn.       
2. Find Your Microsoft 365 Admin: If you’re unsure who the Global Admin is within your organization, you can find guidance on locating them here: How do I find my Microsoft 365 admin? - Microsoft Support 
3. Call Microsoft Support: For additional assistance, use this link to find the appropriate contact number: Customer service phone numbers - Microsoft Support  
4. Contact a Reseller (if applicable): If your Office 365 subscriptions are managed through a federated partner or reseller and the Global Admin cannot open a service request, they should contact the reseller’s support provider. The reseller can assist with initiating the request and coordinating with the Office 365 support team.       

Note: If you’re an end-user, please coordinate with your Office 365 Global Administrator to handle these steps.    

In the meantime, we have found a similar response that may be helpful to you. You can review it at the following link:

Can't upload custom web Add-in in Outlook. The button upload custom - Microsoft Community

Custom Add-in not working in new Outlook - Microsoft Q&A

We hope this information is useful and appreciate your understanding. Please do not hesitate to reach out if you need further guidance. 

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[Note: Please disregard this answer — I've re-posted it as a comment under Jack-Bu's latest reply for better context.]