Many are getting this error due to wrong format of vehicle number.
You have to put vehicle number as AB123B01.
Do not leave any space in your vehicle number.
For example if you number plate is RJ 18 A 123, type it as RJ18A0123 without giving any space.
If the RC book has vehicle number like DL1A123, then you enter as DL01A0123. The vehicle entered in the e-way bill system is only for information and GST officer will accept this variation.