GolamMostafa:
If you want to operate Bluetooth device using this UART port which is occupied by the Serial Monitor, you will face some difficulties which can only be solved by an advanced learner.
Jeez, there is a hell of a lot of nonsense around here, and the above is just pompous claptrap. Translated for ordinary people, it simply means:
"Disconnect bluetooth from pins D0,D1 before uploading your programme"
That is all that separates a newbie from an "advanced learner" in this matter, and should have been in reply #1. Not much, really. This is due to your USB socket and hardware serial pins being internally shared.