Could you please check the below link that might help you:
Let us know if it helps.
A follow up. There is no Library folder in Users/ tom and ElCapitan won't let me create one as it is already in use. I have opened all the Lightroom files to check and/or change the permissions
This is a hidden library please follow the below steps to make changes to it:
- Go to finder > Click on Go> Select Go to folder
- Type ~/Library
- Go to Application Support > Adobe
- Right click on Adobe >>Get info>>Click on the lock>>Enter password
- Under Permissions(at very bottom)
- Give all entries Read and write access
- If you see your user name missing there click on the + sign add it and give read and write access.
Once you give the permissions to the user library then relaunch Lightroom to check whether the error still persist or not.