Can you post a screenshot of the error you're seeing?
good question, I've seen it from other customers and right now we are working on improving documentation for that aspect.
Let me clarify the current behavior. We are throwing notification warnings only if flow would be dropped under current configuration.
You know that each flow carries information about 2 interfaces - Input(Ingress) and Output(Egress) (which is just SNMP indexes).
To be able to store flow to the database, NTA requires at least one of interfaces to be managed by NPM and monitored by NTA same time (I agree that it sounds little bit more complicate that it actually is).
When you receive flow where one interface is managed by NPM and monitored by NTA, second interface is not managed by NPM and option "Allow monitoring of flows from unmanaged interfaces" in ON - you will not receive any warnings and flow will be saved to the database.
When you receive flow from interface that managed by NPM and not monitored by NTA and "Automatic addition of NetFlow sources" option turned ON - this interface will be automatically added to NetFlow sources and flow will be saved to the database.
When you receive flow where both interfaces are not managed by NPM - you will receive notification message and flow will be dropped as we can't process it.
hope this explanation help a bit. Please let me know if I can help you more.