I was trying to send notification using the fcm api "https://fcm.googleapis.com/fcm/send" from postman. Server key was ok and token was pasted fine but was still getting error "MismatchSenderId".
Then introduced the follwoing dependency in the gradle file on android side.
implementation platform('com.google.firebase:firebase-bom:25.12.0')
and it started receiving notifications on the device