Firebase 401 unauthorized error FCM

I don't know if someone uses the [Web API Key] as the [YOUR_SERVER_KEY] for POSTMAN test and keep getting '401 Error'. [Web API Key] is not [YOUR_SERVER_KEY].

You should go to your Firebase console and check this:

to get the correct Server key.

Hope it help.

I noticed from your screenshot that you were using "key: serverKey". Could you try using "key=serverKey" instead?

Also you don't need to have "POST fcm.googleapus.com/fcm/send"; this is not proper json and would explain the error you are seeing. The URL of the request is already defined elsewhere so remove it from the payload.