Hello Zawan Na,
I am a user giving feedback from your link.
1) Make sure you are on the latest S3Bubble WordPress Plugin which is 5.0, and just released today. (looks like you are not 5.0 by the cut-off player display formatting).
2) Create a video thumbnail and upload it to the file folder for this lesson stream in your S3 Dashboard. We just see the default S3Bubble blue swath on white. Ideally you create and optimize all of them in Photoshop. Thumbnail: 1280×720 pixels jpeg for video or audio players. Should be under or no bigger than 125kb. Do it for all your video thumbnails.
I noticed your resolution settings and you have two 720p in the selector display (numbers displayed are funky, but an update to 5.0 should resolve). But, multiple identical streaming resolutions could cause problems and cost more needlessly. Set up your custom hls encoding presets for all different video resolutions and audio qualities and avoid all the others to eliminate duplicates. You can only choose each once.
Other than these observations, your clip played perfectly on a newer Windows 10 laptop on latest Firefox.
Best, James