The Quicktime H264 / PNG Alpha is a common format which you can play both in Windows / Mac Platform.
Most probably you miss to install the video codec to run the video.

Try download and Install one of these players which already contains various video codecs you need:
Quicktime Player https://support.apple.com/downloads/quicktime
VLC Media Player https://www.videolan.org/index.html
Media Player Classic https://mpc-hc.org (for Windows only)

Feedback from buyer :
For anyone using Window, I emailed the file over to my Mac and opened it in QuickTime.
I then re-saved the file in Quicktime and sent it back to the Windows PC, and works perfect.
I remember I had to use this method when using some alpha files from Viddyoze.