Perhaps you failed to answer Yes when TurboTax asked if this was from an inherited account. Edit the Form 1099-R and make sure that you have indicated that it was inherited.
If you answer that the account was inherited and indicate that the decedent had a Roth account before 2020 (actually asking if the decedent made their first Roth IRA contribution for some year prior to 2020), TurboTax will automatically treat the distribution as nontaxable. If you answer No to that question, TurboTax will simply ask you to provide the taxable amount. If answering No does not result in TurboTax asking you to provide the taxable amount, it means that you failed to indicate that the account was inherited.
