Line 12 in the 2019 IRA Contribution Worksheet was eliminated when the age restriction on making traditional IRA contributions was eliminated, causing the line numbers of all subsequent lines to be reduced by 1. However, the reference to line 15 in your case is one of many situations where the numbering in the associated error message was not updated to reflect the changes to the numbering on the worksheet. The error message that you are seeing show be referring to line 14.
This error message implies that you have designated a nonzero amount to be treated as a nondeductible traditional IRA contribution when you have not actually entered any traditional IRA contribution. Either delete the entry on line 14 or, if you have made no traditional or Roth IRA contribution, delete the entire IRA Contribution Worksheet.
