In the case of cellphones, the serial number doesn't apply, you use the IMEI number as a serial number. (It's unique to each individual phone.) Just give them the IMEI number when they ask for the serial number. (Most police departments aren't cellphone repair shops.)
I just checked all the barcodes on my Pixel 2 - IMEI, SKU (which is the make and model, and the same for all Pixel 2s) and eSIM number (which doesn't apply here). There is no serial number. So don't tell them that you can't find a serial number, just give them the IMEI when they ask for the serial number.