Received 1099-C: co-signed for son on student loan. Son will submit on his taxes. Is there spot on TurboTax to make comment regards to this? Or submit taxes by paper?

No,  The form 1099-C,is used to report Cancellation of Debt Income and is taxable.  You needed to enter the amount as other income on line 8 of Schedule 1.  If your son, included the income on his 2020 income tax return, you don't need to enter it. 

In Turbo Tax, you may not enter any comments, but you could write to your local IRS.

If you want to add an attachment to your return, you will have to file by paper mail.  You cannot attachment to TurboTax e-file

Thanks!