Where do I connect a C wire in a Rheem Furnace REAB 1415J?
In thermostat wiring, C stands for Common. Look for a terminal labeled Com, which is another abbreviation for common. 

I did not look up your thermostat, however it is likely that it needs the common wire to work properly. Many thermostats don't really need a common wire but it is almost always recommended to have one especially with these "smarter" thermostats. More information about the common terminal can be found here and here. Some thermostats have a couple of batteries that they can run off of instead of using the common but that would likely not include a wifi enabled one because it uses more power than a simple programmable model.

I hope this helps.