Quick summary

Your catalog file is corrupted and needs to be repaired.

How to fix this

1. Follow the catalog repair steps in this Adobe guide.
2. Try opening the catalog in Lightroom Classic first.
3. If the repair fails, restore a backup of your catalog. Imagen backs up your catalog before each download, and Lightroom Classic saves its own backups as .zip files in the Backups folder next to your catalog.
4. As a last resort, create a new catalog.