Hello Nics
The Webviewer is a sub set of a full browser, so it cannot handle some of the more sophisticated websites. It has been enhanced in later Android versions but that might not make a difference to the viewing of Flash.
How are you trying to view the video? Is it embedded in an HTML file on your device or is it on a website? Who is the owner of the video? If you own it, you can convert into a format compatible with the App Inventor Video Player (such as MP4), which if small can be loaded as media or if larger can be hosted on your own website.