If you can’t find the E series serial number, you can manually set up the E series, avoiding the need for a serial number.
Connect the E series module by USB to your computer. Hold down the MODE button until the status LED blinks dark blue.
Then in the terminal or command prompt window, enter the command:
particle serial identify
Copy the ICCID from the output and go back to https://setup.particle.io and set up an Electron + SIM card (big picture in the upper left).
Even though you have an E series, you can set it up as an Electron using its ICCID which should get around the serial number problem.