It’s based on your answers to the follow up questions and if you made any after tax or non deductible contributions.  Do not put zero in 2a but leave it blank.  What’s it from?  What code is in box 7?

Code J

That is a ROTH.  I think only the earnings are taxable.  @dmertz

The taxable amount is the amount distributed that is in excess of the amount of your basis in Roth IRA contributions and conversions in your Roth IRAs.  The calculation of the taxable amount is performed on Form 8606 Part III after you have answered the questions that appear after you click the Continue button on the Your 1099-R Entries page to indicate your basis in Roth IRA contributions and conversions.  The gross amount of the distribution will be included on Form 1040 line 15a but only the taxable amount calculated on Form 8606 will be included on line 15b.