Problem with the microphone.

Probably best to start with a little more information, like what the input is and what OS you are using. A screenshot of your REW soundcard preferences may help. That message appears when REW asks the OS for a connection to the input device you have selected and the OS refuses. That could be because it doesn't support the requested sample rate, or it could be because some other application has taken exclusive control of the interface. Sometimes the OS has got things mixed up and a restart restores normal behaviour.