Dear Jim
Problem solved by following these instructions (from another forum):
Solution 5: Set correct permissions on Lightroom folder inside Application Support
The Lightroom folder inside Application Support could have the wrong permissions preventing Lightroom from writing to it which can cause this error.
Navigate to the following folder: /Users/[user name]/Library/Application Support/Adobe/Lightroom
Highlight the Lightroom folder and chose File > Get Information
Under Sharing & Permission, verify that your have Read & Write Privileges. If not, click the padlock icon at the lower right, enter your password when prompted, then click the Read only entry and select Read & Write.
Thank you for trying to help!
Susan