I just filed my taxes, and realized I did not include a 1099-C that I got. How can I fix this?

Once a tax return has been filed it cannot be changed, canceled or retrieved

You will have to wait for the IRS to either Accept or Reject your filed federal tax return.
If the return is rejected you can make the necessary changes and e-file again.
If the return is accepted you will have to amend your original tax return. An amended return, Form 1040-X, can only be printed and mailed to the IRS if the original tax return was not e-filed. The IRS will take up to 20 weeks or longer to process an amended tax return.

Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.

See this TurboTax support FAQ for amending a current year tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-current-year/L7eS6o1qh_US_en_US?uid=lfunevhk

It depends. If your return does get accepted, then you will file an amended tax return. If your tax return should get rejected for some reason, you can add the 1099-C and then resubmit your tax return through e-file a second time.

• When to use Form 1099-C for Cancellation of Debt?
• How do I enter a Form 1099-C?
• How do I amend my federal tax return for this year?  (Be sure to review before you begin.)