It might be a permissions problem - can you try uploading with Administrator privileges? This post here seems to say that you might need those.
I've also heard of problems with Scratch for people using parental monitoring software on Macs. If you have that installed, can you try turning it off and seeing if that allows it to work?
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You don't need administrator privileges unless you actually manually set it yourself. Scratch beforehand does not require administrator privileges.
On my new computer I"m admin...yay!

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