IRAs do not have a cost basis but instead may have basis in nondeductible traditional IRA contributions. IRA custodians have no way to know whether you made nondeductible traditional IRA contributions or not.
To enter you basis in nondeductible traditional IRA contributions made prior to 2021, click the Continue button on the page in 2021 TurboTax that lists the Form 1099-R you've entered, answer Yes when asked if you made nondeductible traditional IRA contributions, then enter the amount of basis from your last-filed Form 8606 (usually line 14).
