Nanoflows (client side) can call Microflows (server side) for good reasons (complexity) you can all pass on non persistent object in the call.
If you want to send the persistent object you have to synchronize.
Full sync: https://docs.mendix.com/refguide/synchronize
Partial sync https://docs.mendix.com/refguide/synchronize-to-device
Still I get Error.
Wise men and the errormessage are saying: When calling a microflow from a nanoflow, don’t pass a persistable object!
Maybe a solution is to create an extra nonpersistable entity EventNP, pass that along to the microflow, in the microflow copy EventNP to Event (Singular as all entity names should be singular) and make that microflow call your current microflow.