Open the location of the catalog. The default location of the catalog would be File Explorer > Pictures > Lightroom.
While opening a catalog, you are provided with the following error.
To solve the issue:
- Ensure that you have admin privilege in the machine.
- Restart the machine and then open Lightroom Classic.
 
If you still see the same error, do the following:
Windows
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Open the catalog-name.lrcat-data folder. 
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Locate and delete the "CURRENT" file. 
 
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Open Lightroom Classic and ignore the same error message that may pop-up. 
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Re-open Lightroom Classic again. 
Mac
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Open the location of the catalog. The default location of the catalog would be in the Pictures folder. 
 
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Right-click the catalog-name.lrcat-data folder and select show package contents. 
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Delete the "CURRENT" file. 
 
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Open Lightroom Classic and ignore the same error message that may pop-up. 
 
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Re-open Lightroom Classic again. 
The above procedure should solve the issue. If you need further assistance, try our community page or check Learn & Support.