My wife is self employed, age 57, and contributed $24,000 to her SEP IRA in 2017 that was full adjusted for the AGI. Using Turbo Tax for 2018, it says her limit is $6500. What changed in the tax code OR is Turbo Tax not treating SEP IRA's properly now.
$6500 is the limit for deductible contribution to a traditional IRA in 2018 for individuals 50 years old or older. Are you entering the contribution as part of business section (shows up on the Keogh/SEP/etc worksheet), or the personal section (shows up on the IRA worksheet)? In step-by-step for “personal”, TT asked me about contributions to “Traditional IRAs”. I answered (correctly, IMO, since SEP IRAs *are* considered traditional IRAs) with the amount I contributed to my SEP IRA, and this caused TT to apply non-SEP limitations, and create a form 8606 for non-deductible contributions. The solution was to enter 0 in that section. The AGI reduction should show up on your schedule 1 as a SEP adjustment, not an IRA deduction.
Nothing has changed in the tax code or in TurboTax with regard to entering SEP contributions.
TurboTax's Traditional and Roth IRA Contributions section under Deductions & Credits asks for your regular *personal* contributions to traditional IRAs. However, SEP contributions are employer contributions, not regular personal contributions, so they are *not* to be entered under Deductions & Credits. A self-employed SEP contribution is instead entered in the Self-employed retirement section explicitly as a SEP contribution.