Your download was probably corrupted. Did you run a hash check to insure download file integrity?
If you look on the download page
you see it has a Sha 1 hash / checksum number.
You can use that to check that the file finished download correctly.
I use a program called MD5 & SHA-1 Checksum Utility
You can run that to check your download .iso file and it should match the hash number for your download version.