Videos don't play in browser either

I know this topic already exists with this very same title, however the same problem happens to me and the solution presented didn't work for me.
I'm using opera 57.0.3098.91 on debian stretch and the presented solution in the original topic does not apply to my case, the flag "Dynamic demuxer selection" on opera://flags/#dynamic-demuxer-selection cannot be enabled, because when trying to enable this flag, the message "Not available on your platform." appears.