Things to try:
1. Download again. Sometimes the download is corrupted for no obvious reason.
2. Download somewhere else and copy the downloaded file to your USB-stick. Then install from that.
3. Contact the application vendor.
I try to download casino games and it keeps telling me "this installation package could not be opened. Verify that the package exists and that you can access it, or contact the application vendor to verify that this is a valid window installer package" so what can i do to fix this?