Does this happen the first time you try to connect to your camera, or when you try to rerun your code?
The issue is something is using your camera already, which prevents MATLAB from connecting to it. You will need to close whatever is using it first. If it is another program on your computer, close that program.
If it is happening the second time you run your code, it is likely you have not properly closed the object from the previous run of your code. Be sure to clear your camera object. See the recommended steps here.
% create camera object
cam = webcam
% <your code>
% At end, clear 'cam'
clear('cam');
