Do not what?jake37 said:The message is likely your device not supporting the codex (encoding) of the video. Videos can be encoded in many different format; actually it is more complex than that - there is the metadata which describe the video and than the actual video itself. An example of the metadata encoding is (for example) avi and an example of the video encoding (for example) is hvec.
Here is a sample article that talks about the different types of encoding that can be applied to a video:
Anyway the simple answer is your device does not support the video file you are trying to view. This can be resolved in some cases (depending on your device) by downloading a package or browser plugin to add support.
Most browsers have a method of showing the file format you are trying to load - or you can check the extension on the file (the extension on the file is not definitive answer but it might provide a useful clue.