This tends to come up with New York driver's licenses, so I am going answer assuming you are in New York. The document number is an 8 or 10 digit alphanumeric number found either on the back of the license or lower right hand corner (depending on the year of your license). It is not the same as the drivers license number. I have included a link to the NY authority that displays images below.
**Some licenses depending on issue date don't have these document numbers on them. Review the link above, but if you do not have a document number, then you can just skip it. You should be able to hit continue without actually entering any of the license information (leave it all blank). You return will still be processed without it as the IRS does not require the license number and neither does NY, but they are asking for it more of an identity protection measure (some states like Alabama and Illinois are requiring it).
