There are a couple of different use cases where this error might occur. Please see below for the case that most closely matches your intended use.
Case 1: Using the same camera name for two cameras at the same time
This is not an option, as each camera in MAX must have a unique name. The recommended workaround is to iterate on the camera name in a predictable manner. (eg. camera1, camera2, etc.)
Case 2: Develop and deploy identical systems with identical camera names
If you have already deployed the system with the camera name you want to use, or no longer have a need for the previous camera with that name, you can free up the name by deleting the .iid file for the camera. These files can be found in the following location for Windows:
After deleting the .iid file, close NI MAX. When you open MAX again you can rename the camera.