How do I attach my 1099-K to my amended return? IRS could not process my 1040X without supporting evidence of withholding reported on 1099-K.

[DMarkM1]

Did you get a rejection email or a letter from the IRS asking for the supporting documentation?  If you received a rejection email, then you will need to print/mail your 1040X along with any forms that changed and the supporting documentation such as the form 1099K.  You will mail the forms to the IRS Service Center that handles your state.  When you select the print/mail option in TurboTax the mailing address should be included in the printed forms.  Also you can use this link to find the correct mailing address if needed. 

If you received a letter from the IRS, then the letter will have instructions on where/how to send the supporting documentation only.