How to find IMEI serial number on a WiFi Only Device
The settings app has a general section where you can get all the various MEID / IMEI / ICCID and serial numbers. If you are being forced to make up a number, perhaps the listing isnt correct for a WiFi only device or you need to find a seller that can properly classify your hardware as WiFi only. 

If you did have a cellular capable iPad, and you can’t find the information in the about section, it might need a restore or repair.