(This almost certainly has nothing to do with an excess contribution.)
Because you have basis in nondeductible traditional IRA contributions, the year-end value of your traditional IRAs is needed for TurboTax to be able to properly calculate on Form 8606 Part I the taxable amount of your IRA distribution. Until you make that entry, TurboTax will assume that all of your basis in nondeductible traditional IRA contributions can be applied to the distribution, when, by law, only a proportional amount of that basis is permitted to be applied. Once you enter the year-end balance, TurboTax recalculates to determine the correct, higher taxable amount of your IRA distribution.
