They want the "Flight Controller Serial Number". That is what I've been using for years and it works every time. To retrieve this, follow these instructions:
1 - Power up the DJI Go 4 App.
2 - Power up your aircraft.
3 - Once they connect, clear any errors/warnings, so you are ready to fly.
4 - Touch the three dots in the upper-right corner:
View attachment 145838
5 - Scroll down to the bottom of this screen and touch "About":
View attachment 145839
6 - You'll find the "Flight Controller Serial Number" here, right after all the version info:
View attachment 145840
At least, this is where I get the number that works on my Mavic Pro Platinum. I've made notes of all of my serial numbers, and have it on a notepad file I open when I go to the DJI site.
The serial number on the drone (under the battery on my MPP) will NOT work here. At least it has not worked the last time I tried it.