Most of the 'answers' that I have found are not well explained (at least to me, since I have very little understanding of programming, and of 'snap') I did check to be sure that a 'snap' directory is in my 'path'. And one suggestion was to install Chromium, so that Opera could make use of one of the supporting files for Chromium. I did that, but ran into an error installing it (something about python3). Never-the-less Chromium seems to be working fine, but Opera still won't play certain files (for instance mp4).
On the same hard drive, I had previously installed Ubuntu Mate 18.10, with Opera. Then when 19.10 became available I installed it, reformatting the partition for "/", and using the same data space for "/home", in order to preserve previous data. Is it likely that some vestiges of previous installations of Opera, or python are hidden somewhere in that data space conflicting with the new installation? If so, what should I be looking for, or how to I eliminate them?