Solved this way.
We just had this issue as well - the Hue light that came with the Plus was not discoverable. We tried the reset with serial number and different lamps. Then I found the post in this forum about resetting the Echo Plus. That worked!
Caution!!!! Reset will likely discard you’re current Echo Plus settings and you’ll have to start over. Ours was new so we didn’t care.
Reset by turning off the Plus and logging out of the app. On the bottom of the Plus there is a tiny hole. Use a toothpick or bent paper clip, insert into the hole depressing the button for about 12 seconds. The Plus will beep to signal reset is complete. Restart the Echo Plus, log into the app and setup the Echo Plus. Now discover devices and your Hue light will hopefully be discovered!! Hope this helps.