The account was closed in 2019 but I still have to report it. Since the account was closed, I no longer have access to anything online and the only copy I have is Copy C. Can I send this in instead of Copy B?
Yes, but only if you are mail filing and there is tax withheld in box 4.
You only attach copied of 1099 form to a mailed tax return if there was federal tax withheld. With e-filing nothing is ever sent reguardless of withholding or not - it is already sent electronically.