There can be several reasons why Logger Pro will fail to communicate with the Watts Up.

Logger Pro 3.8 or newer is required for Watts Up. If you have an older version, you will need to update. See:
https://www.vernier.com/support/updates/logger-pro/

The blue USB cable shipped with the Watts Up should always be used when connecting to a computer. Other USB cables often have connection issues due to physical interference with the Watts Up housing. The blue cable included has a small but crucial extension of the silver connector on the cable to allow secure connection to the device.

Communication with the Watts Up via USB uses a special software stack and driver to enable serial communications. The driver on Windows is the FTDI driver. Modern versions of Logger Pro for Windows installs the needed FTDI driver. You may need to upgrade to the new version of Logger Pro. The updater is a free download from our web site at:
https://www.vernier.com/tech/lpupdates.html

Logger Pro for Mac does NOT install the FTDI driver; the Mac version of Logger Pro interprets signals to identify the Watts Up. Note that if an FTDI kext (kernel extension) is installed on the Mac by another program, it may interfere with Logger Pro‘s communication with the Watts Up. In that case the interfering kext must be removed or temporally disabled in order for Logger Pro Mac to communicate with the Watts Up.