According to the IRS instructions, If you aren't required to file an income tax return but are required to file Form 8606, sign Form 8606 and send it to the IRS at the same time and place you would otherwise file Form 1040, 1040-SR, or 1040-NR. Be sure to include your address on page 1 of the form and your signature and the date on page 2 of the form.
Since you already filed the return but did not include the 8606, you can send it in as a stand alone document. If you sent a 8606 in but if it is incorrect, I would advise filing an amended return that includes the corrected 8606 just to be safe.
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