The documentation says it's possible to send test notification:
That whole notification section is not familiar at all. There is current work to completely redo Monica and when I view their beta at https://beta.monicahq.com/ then it seems to reflect that new version. So I suspect that documentation is wrong for the current version (which hasn't been meaningfully updated in years).
In addition to what rhe other user asked, what do logs say, how are you configuring emails?