There is no way to enter Section 1291 interest in TurboTax Online. It can only be entered in forms mode. Forms mode is only available in the CD/Download TurboTax software, not in TurboTax Online.
Furthermore, TurboTax does not support Form 8621. You would have to fill out Form 8621 yourself. Since Form 8621 is not included in TurboTax, you would have to file your tax return by mail in order to attach Form 8621.
To enter Section 1291 interest in the CD/Download TurboTax software, click the Forms icon to go to forms mode. Open the "Form 1040 or Form 1040-SR Worksheet." Scroll down to Schedule 2 Part II. In the Additional Other Taxes Smart Worksheet, enter on line O the amount of interest from Form 8621 line 16f. TurboTax will then check box c on Schedule 2 line 8 and enter the code 1291INT and the amount.
You might prefer to seek professional help with this.
