If it helps at all, the new sync system in Codebook 4 was just rolled out in December of 2019. That would be the earliest you would have been prompted to create a backup of your Sync Key via either QR code, or written word list.
Before you do that, can we confirm that you do not have Codebook on any other platform, or possibly an old backup of the strip.db file on Dropbox?
If you have determined that you do not have any previous backups of your data, nor any backups of your Sync Key, you would need to reset your local environment. We have steps for doing this on all platforms here. We would recommend backing up your Sync Key in the future as this will prevent you from having to start from scratch if you have a backup of your data, even if you forget your master password.