I'm not a mac - specialist, but...
Do you have full rights to the location which the catalog is saved?
What is the whole error message?
I get 2 error messages. The first box says - An internal error has occurred, (cannot create the required folder:/users/Lynn/Library/Applications Support/Adobe/Lightroom/Desktop Presets)
the other box says - An error occurred when attempting to change modules.
I used to have elements on this machine but I have uninstalled it now and all my photos are out of the elements organiser.
LR opens fine on my macbook and will open on my husbands user account. I have full admin rights on this machine.
You don't seem to have enough permissions at the location below:
/users/Lynn/Library/Applications Support/Adobe/Lightroom/Desktop Presets
Please make sure that your user has full read and rights permission to the directory given above.