Well this is new... In adding new folders or assets to my various .xcassets items in Xcode 9, it adds the item a-ok but always gives me a sheet stating:
You don't have permission to save the file "some file" in the folder "Assets.xcassets". To view or change permissions, select the item in the Finder and choose File > Get Info.
I have full read-write access to the entire drive that my projects reside on. I also double-checked the individual items (top-level folder of 'Assets.xcassets') and everything inside it.
Anyone else seeing this? Yet another strike against version 9. I will shortly try the 9.1 beta and see if that's any better. This is definitely causing a brick wall since I need to add numerous assets and will get this goofy sheet for every single one of them.