The error message indicates that you have a malformed catalog.
Recovery steps depend upon your Catalog’s current Sync Status.
If you believe you are fully-synced, or if you have yet to place any assets in Lightroom Desktop, the local MCAT file can be renamed, allowing for the sync of your catalog information down from the cloud, restoring your local catalog.
Make sure you have a good internet connection
Locate the MCAT file and rename it from “Managed Catalog.mcat” to “Managed Catalog.old”.
Users>username>AppData>Local>Adobe>Lightroom CC>Data>Catalog ID string>Managed Catalog.mcat
Right-click on file and select “Show Package Contents”
Navigate to: >Catalog ID string>Managed Catalog.mcat
Allow Lightroom Desktop to fully sync
The Lightroom catalog should now sync from the cloud to your local desktop. This may take some time. Checking with the Cloud icon will update you on the progress. When complete, you will have a new “Managed Catalog.mcat” file. Retain the old MCAT file until you are satisfied of the recovery then the Managed Catalog.old can be deleted.
If the Lightroom application is not completely synced there are two scenarios:
All original images were imported from Lightroom Mobile, Lightroom Web, or were synced from Lightroom Classic as Smart Previews.
Follow the Synced instructions to restore your Lightroom Desktop catalog
Some or All original images were imported via the Lightroom CC desktop application.
Locate the originals:
· Navigate to: Users>username>AppData>Local>Adobe>Lightroom CC>Data>Catalog ID string>Originals