Do you have 7zip installed? If yes, then pheraps the download is corrupted. Is it reproducable if you try again with a new SourceTree download?
SourceTree comes with the command-line 7z.exe inside it so it can extract the embedded Git archive. Please do try downloading again as the archive seems fine here - also please check whether you have any proxy servers (e.g. corporate network) which may be intercepting your download.
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