It probably means that your video card doesn't support hardware acceleration or that Opera can't use it.
And Opera depends on the OS and GPU to play html5 videos.
What about opera:media-internals? Does it shows something?
Are Opera not updating the portable version then?
AFAIK that portable version was never official.
Development of Opera Presto (Opera 12.x and older) stopped about three years ago except for a security update about two months ago.
Regarding the issue, have you checked if author mode and/or content blocker aren't enabled by chance?
You can download Flash and I have done that. You can get it from Adobe's website and I have done that. But when I install it it is not recognised by Opera. Is it because I am running the portable version? I doubt it. The button is the big yellow install one that you see when you go to Adobe's site, except that I see it only for a split second then it disappears.
It works in Chrome, just not in Opera. What's going on?