If you are a first-time filer, entering "0" is the correct entry. What I would recommend is to go back to your AGI entry and clear the box. Try entering "0" once more and ensure there are no extra spaces before or after your entry, as this may throw off the IRS system.
If you continue to receive a rejection for this same reason, you may have to file a paper return. Please see the TurboTax FAQ below for more information regarding AGI E-file Rejections.