Are you getting an error message when you try to e-file by leaving the box blank? You should be able to leave it blank if your W-2 doesn't have one and e-file. Make sure you are not inputting anything into that box and then submit.
**If you are still receiving an error message when left blank, comment with error message here.
This is something new the IRS is testing out for 2018, so not all W-2’s will have one. Please see here IRS Expands W-2 Verification Code for Filing Season 2018 for more on verification codes on the IRS’s website. Per IRS: "Taxpayers without a verification code on their Form(s) W-2 may leave the verification code box blank in their tax software product."
What if the verification code box on my W-2 is empty?