Does the agent still install install even though you got the error?

If so, that's a known quirk of the MobiControl agent. The installer will attempt to write the CAB file to the device even if the CAB file already exists. Windows will show a dialog box when it fails to overwrite the existing CAB. In most cases this is not a real problem as all that matters is that the agent installs.

If the agent is not installing, then you may have an issue with the device or the Agent was not built correctly. I recommend contacting the support team and working with them towards resolution.