This error occurs as Lightroom do not have permission to write files to the location /Users/Username/Library/Application Support/Adobe/Lightroom
Please refer to the Marked answer in Duplicate Threads below
Do let us know if that helps
On the menu bar click Go whilst holding down Opn(Alt) key and choose Library. Note that the user library is hidden until you press Alt.
Navigate to Application Support and then Adobe and highlight the Lightroom folder. Press Cmd+i and ensure everyone has read and write privileges. If not click the padlock icon and change.
Close the Lightroom info dialog and re-launch the application.
The full file path will be:
Macintosh HD> Users > samuelhesketh > Library > Application Support > Adobe > Lightroom
You may also need to check permissions on the sub-folders: