How do I attach a windows "csv" file to my amended 2020 tax return

It cannot be done ... if you need to attach something you must print, ink sign and mail in the return. 

If TurboTax won't accept your CSV file, proceed as follows.

Adjust your csv files so the columns are in the same order as on Form 8949.

Put a header with your name tax year and SS Number

before including with your amendment.