This solution didn't work for me but this one did http://www.sharemuch.com/2014/03/24/how-to-resolve-server-error-this-file-may-not-be-moved-deleted-when-working-with-sharepoint-master-page
-Copy the existing *.html file in the gallery -Rename the copied *.html file to the name you wish to give (or have already given) to your custom master page -To delete the masterpage, delete respective *.html file instead, etcSteps from the above blog:
- Copy the existing *.html file in the gallery
- Rename the copied *.html file to the name you wish to give (or have already given) to your custom master page
- To delete the masterpage, delete respective *.html file instead, etc
