You are not able to download because the author who created the workbook has not given the download access. Tableau public workbooks can be downloaded only if the the author has enabled download option
Hope this helps!
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Manoj is correct. Authors on Tableau Public can set a viz to hidden, meaning visitors to their profile can't see it on the thumbnails. However, anyone that knows (or stumbles across) the URL will be able to view the viz. Further details in the FAQ here...
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