Please read the following solution, it helped me out: https://askubuntu.com/a/1091454/242730

# Check if you are able to open HTML5 H.264 videos
    https://www.youtube.com/html5

# If not - do the following
    Get "0.35.4-linux-x64.zip" or newer version from:
     https://github.com/iteufel/nwjs-ffmpeg-prebuilt/releases/

# Extract "libffmpeg.so" from archive
    > unzip 0.35.4-linux-x64.zip

# Create directory "lib_extra" in opera and copy "libffmpeg.so" there
    > sudo mkdir /usr/lib/x86_64-linux-gnu/opera/lib_extra
    > sudo cp libffmpeg.so  /usr/lib/x86_64-linux-gnu/opera/lib_extra

# Restart Opera and check YouTube HTML5 link again
    https://www.youtube.com/html5