You may have a schedule or form included in your return that is not supported by the free program.
TurboTax Free only covers simple 1040 returns with no schedules other Schedule EIC (for Earned Income Credit) and Schedule 8812 (for Child Tax Credit). If you're itemizing, reporting rental income, stock sales, or most 1099 income (for example) you would need to upgrade.
Click here for more information on what's included in TurboTax Free.