You can leave it blank. The driver's license number is an added security measure the IRS and states have put in place for 2016. 

Your return can be filed and will process without your drivers license number.  

Please see FAQ below for more information. 

In an effort to help protect your identity and fight tax Fraud, the IRS and State tax offices are now asking that tax filers provide their driver’s license or state issued ID number. This step is not required in order to file your taxes, for most states, but could help speed up the processing of your return and will not affect your return’s submission. Omitting any of this information may delay your return and refund processing. 

States such as Illinois and Alabama do require an ID to be provided in order to e-file this year. 

For additional information, you can view the IRS information on the New Security Safeguards. or to see if your state is participating, please contact your state revenue agency.