I figured out my issue. I didn’t have enough ZigBee devices in my system that we’re plugged in to a power source and acted as repeaters/extenders. While the number of ZigBee devices you can add is high, you have to have enough powered devices to support them. The SmartThings support folks told me each powered/repeating devices could support only 40-50 devices. I forget the exact number. Anyways, I bought a few more ZigBee powered outlet switches, spaced them around my house, and added them to the network. Then, I unplugged the hub and took out the batteries for 15 minutes. After restarting the hub, I was able to add the MCT-340E sensors without a problem.
See this thread: