"ABP" is an amortized bond premium. This means you purchased a bond at a certain price above par. For example, you paid $1870 for an $1800 bond.
Did you import your 1099-INTs or enter them manually? In either case, did you have an entry (even a $0) in Line 11 of either 1099-INT?
If you had a positive entry (>$0) on Line 11 of your 1099-INT, you will have an ABP interest adjustment. The TurboTax program will prompt you to upgrade since this adjustment cannot be calculated or reported using TurboTax Free Edition (Online).
If you had a "$0" on Line 11 of your 1099-INT, you don't have an ABP interest adjustment, but the program thinks you do. You would have to delete the Form 1099 and re-enter it in the 1099-INT interview - any Lines with a "$0" amount should be left blank (i.e. don't type in a "0").
See our AnswerXchange article How do I delete forms in TurboTax Online?
If you did have a positive number in Line 11 of either 1099-INT and it is a very small amount (e.g. <$10), you may decide to remove the amount (using "Delete a Form" tool); this will not change your tax return in any material way and would allow you to avoid the upgrade.
