If you can view your return, you should also be able to print all of the forms filed with your return.
I'd copy your entire Schedule D, including all forms filed with the IRS, and mail as supporting documentation. The notice you received from the IRS contains instructions on how to reply to them, as well as a time deadline in which to reply.
As long as you mail documentation that supports your claim that your return is correct, you should be good. I have no explanation as to why the forms the IRS says they received were incomplete, but you have the completed forms.
Be sure to include a cover letter when you respond to the IRS that explains what's included in your mailing. Additionally, mail your return by certified mail so that a signature is required on receipt.
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
