Same as CodyBrooks. Got this error trying to start install on external SSD, only was able to continue after formatting as APFS.
drekcheck
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I'm seeing same thing in my own app, and tried Photos as well with same behavior.
Repro
Open Photos
Select a Photo
Share button (lower left)
Scroll to "Export Unmodified Original"
Select Export Unmodified Original
See "Save" button dimmed.
Try to create a new folder -> Doesn't work