The level editor will automatically save the level in the folder path where app is located on your hard drive.
You need to make sure that you have write permissions to that folder.
I'm not a Mac guy myself, but I believe you can find some info on how to do it here for example: https://support.apple.com/guide/mac-help/change-permissions-for-files-folders-or-disks-mchlp1203/mac
(If you are unable to set the write permissions for the particular folder where it's currently located, perhaps you can move the entire app to another folder on your hard drive).
Hope that helps!