You should still be able to view some videos depending on how they are encoded. To fully support HTML5 you may need to make sure you have packages like FFmpeg installed. Patent and copyright restrictions are among reasons why many Linux distros do not come with packages such as FFmpeg in a new install as it provides codecs needed to fully support HTML5 players being used for video, audio and even games now instead of using the Flash Plugin. Adobe is ending all support of their Flash Player at end of 2020. As you are using a flavour of Ubuntu so perhaps see https://linuxconfig.org/ubuntu-20-04-ffmpeg-installation or https://help.ubuntu.com/community/RestrictedFormats to help you get FFmpeg installed.