You cannot use TurboTax Online to amend a tax return that you filed with the CD/Download TurboTax software. If you purchased the disk from TurboTax.com there might be a 2018 download in the account that you used for the purchase. See the following FAQ for instructions to access it.
If not, you can use the 2018 amend software, which you can download from the following link.
In either case, you will need the .tax2018 file for the original tax return that you filed.