You cannot simply total the nondeductible contributions that you failed to report and treat that as an adjustment to your basis. You were required by law to report the nondeductible contributions on Form 8606 for each year that they were made. You must file each Form 8606 that you were previously required to file. Box 14 of each Form 8606 will carry forward to box 2 of the next Form 8606 that you were required to file.
Had you entered the traditional IRA contribution and TurboTax determined that it was to be treated as nondeductible, TurboTax certainly would have created the necessary Form 8606. Check your previously filed tax returns to make sure that they contributions were not actually deductible and present on Schedule 1 line 20 (or as an IRA deduction on Form 1040 for years before 2018, the year Schedule 1 was introduced).
