Depending on your bike model there are a few different locations where the serial number may be. 

Head Tube

On a large majority of Biktrix eBikes the serial number is located on the front of the head tube. Notable at the bottom of this where the fork inserts into the head tube.

Some of the exceptions - and other places to check for your bike are located below.

Juggernaut Ultra Beast and Monte Capro Ultra

On the Juggernaut Ultra Beast and the Monte Capro Ultra the serial number is located on the motor casing behind the seat tube.

The first image shows the Juggernaut Ultra Beast Serial Number location.

The second image shows the Monte Capro Ultra Serial Number location.