Because a 1099-C requires a schedule that is not supported in the Free Edition.
The TurboTax Free edition only supports simple tax returns that do not require any additional schedules. More complex tax returns will require Deluxe or higher depending on the forms that are needed.
You might be able to use the Free File (formally called Freedom Edition) for incomes under $36,000 and supports all states and most forms.
The Free File is part of the IRS Free File alliance and is totally separate from TurboTax website.
You can also access the Free File tax alliance programs via the IRS website.
https://apps.irs.gov/app/freeFile/
https://freefile.intuit.com/
How do I switch to TurboTax Free File?
https://ttlc.intuit.com/replies/3491786