It sounds like your first attempt at an OJS 2.4.8 to 3.0 upgrade failed, and you made a second attempt, with the result being an error message that you quoted above. This means your database is in an indeterminate state between OJS 2.4.8 and OJS 3.0, but without the error message from the first attempt, it’ll be hard to tell exactly what state your database is in.
If you’re absolutely sure you don’t have a backup, and don’t have the error message from the first attempt, your only option may be to look at the upgrade steps in dbscripts/xml/upgrade.xml and see if you can determine from the presence/absence of data what step was last completed.
This situation is exactly why we strongly recommend making a backup before upgrading.
Regards,
Alec Smecher
Public Knowledge Project Team