Schedule 1 has many different things. Is the IRS asking for more information?
"schedule D is incomplete or missing from your return. Complete the schedule with information that supports $x,xxx.xx on line 13, Schedule 1. Also attach all supporting forms or schedules, as required."
Line 13 is capital gain or loss. Is that what you reported? If you had a 1099-B with security sales the you should have had a schedule D. If you print all forms and worksheets is there a schedule D there? The 1040 Schedule 1 has a check box next to line 13 that is checked if schedule D is not required - is it checked?
Yes, I reported a loss. There is a schedule D there. The box is not checked, but a schedule D was included. Are they asking for proof of how I got the #'s entered?
Your proof would be the brokerage documents (1099-B) that you used to enter the data from. The IRS should already have those. You can call then number on the letter and find out just what more they want.
the IRS is probably asking for form 8949 which details your security sales. this is required if basis on some or all sales not reported to IRS (this should be indicated on the 1099-B)
or if basis was reported you didn't use lines 1a and/or 8a.
or you didn't use the same basis that the brokerage reported which requires and explanation.