If a Mac Dropbox user is reporting problems with Dropbox not being able transfer files due to a permission error, it’s likely this can be resolved using Dropbox’s hidden Fix Permissions feature.
To access this Drobox Fix Permissions feature, click on the Dropbox icon near the date and time, click the gears icon, then select Preferences. Next click on the Account tab, and you will see a button named “Unlink This Computer”. Hold down the Option key, and the “Unlink This Computer” button will turn into “Fix Permissions”. See if clicking this “Fix Permissions” button resolved the error.