Did you make a payment with the extension? Or say you would? Check schedule 5 line 71. Maybe it thinks you paid one.
There was also this......
If you filed early and had Schedule D for investments or sales in your return the IRS had an update.
IRS info on Schedule D changes (after May 15, 2019)
https://www.irs.gov/forms-pubs/error-in-tax-calculation-in-schedule-d-tax-worksheet-form-1040
