If you are using an IRTrans USB module, you need a "driver" to make the OS recognize the module. These are provided by FTDI at their website for VCP drivers.
The latest driver is Version 2.4.4, here's the direct download link.. This version is 64-bit only, so if you are running an older system don't install this driver!
After download and installation please reboot your Mac - unfortunately this is necessary.
macOS BigSur is not supported by this driver. We are working on an update to iRed2 which does not require a driver anymore