If I got a letter from the IRS asking for a 1099 form to verify withholding amount reported on my tax return. can I fill out a 1099 form for myself if self-employed?

I sell items online and do not work for anybody to receive any 1099 or W2 but the IRS wants one of the 2 to prove the withholding amount that is on my tax return that I e-filed. I'm not sure how to provide that's unless I filled it out myself however this is my first year filing as self-employed and I'm unsure how to move forward here.