You may have multiple block instances pointing to the same device in your model, another Simulink model running, or you may similarly have run or created instances of audioDeviceWriter in your MATLAB session. Checking other models open, other blocks in your model, or clearing (or even releasing) existing competing object instances in MATLAB should solve this.
Please refer to the code in the example "Audio Stream from Device to Device" under " Quick Start Examples" in the page Real-Time Audio in MATLAB. In particular, the few lines of code below should do what you need:
% In the two lines of code below, provide any additional property value necessary to point the object to your sound card(s)