An issue in Final Cut Pro 10.6 has been reported where it sometimes shows an error message “The XML document could not be imported. DTD validation failed. (The data couldn’t be read because it isn’t in the correct format.)”
tThe work-around is to go to System Preferences > Language & Region and temporarily set the Region popup menu to “United States”, them import the XML file again.
After import you can reset the Region to your preference.