Codec is what makes compression and decompression of your files possible. Because of this process, a single video can be played across multiple devices. These Codecs convert your file into a compatible format so that it can be played on your system. In case, you get Conversion failed: A required codec could not be found or failed to install, that the Plex doesn't have the code required to convert to make the processed media playable. To resolve the issue, update Plex Player to the latest version. If the latest update doesn't incorporate the required codec and you desperately need it, install the codec manually onto the log files of Plex.