The error means that the database has been corrupted. If you have a recent backup of the database, you can copy that over the existing database.
If not, you'll have ti log out of your Kobo and log back in. That will recreate the database. You will lose everything on your Kobo and you'll have to put all of the eBooks back on. Before you do that, you can copy all of the eBooks to your computer and once the database is fixed, you can copy them back and then once the eBooks are populated in the database, you can plug your Kobo into the computer with Calibre running to fix your collections/series/metadata.
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