It's probably more common that your MAGI for the purpose of a Roth IRA contribution is less than your AGI because the calculation of this MAGI involves subtracting from AGI the amount of any Roth conversions from a traditional IRA or rollovers from a traditional account in a qualified retirement plan to a Roth IRA. If you've moved at least $16,000 taxably to a Roth IRA in either of these ways, that would bring your MAGI below the $196,000 for 2020 where the phase-out range begins.
If your MAGI ends up being above $196,000 and you enter a Roth IRA contribution of the maximum permissible for your or your spouse's age, TurboTax will tell you the excess contribution.
You can see your MAGI and the maximum Roth IRA the calculation on TurboTax's Roth IRA Contribution Limit Worksheet ("Roth IRA" in the forms list).
