Problems creating an Add In for Outlook

Question

I can't load an Add-In for Outlook via manifest.xml, always getting a "not allowed" error. The manifest.xml is correct and is already minimal and as simple as possible, it has already been validated via the npx office-addin-manifest validate command, and it has also passed the installation test itself (with the error appearing later). We have already tried installing the manifest.xml both locally, via Outlook web, and now via the Microsoft 365 admin center, all without success. I load the manifest.xml from a local file, not from a web address, and at this moment we are focused on installing it via the Microsoft 365 admin center, since that is the most comprehensive option, in an account that is a corporate trial version of Office 365 E3 (in Teams), where I am the administrator and only user. We think it must be some issue with rights or some account limitation, but we have no idea what it could be, can anyone help us?