Thats a simple Issue with default template of MyBB.
Its a bug within MyBB that needs to get solved.
Here is a fix i can suggest :
The error is due to the reason that you are trying to run the Video with http:// rather than https:// which should be the actual protocol.
So to fix this navigate to your theme templates > Ungrouped templates > video_youtube_embed
Then replace the code :
There are 2 instances where the code appears and you have to edit both of them.
This would solve the issue.
This is fix for not only Tesladark but with Every MyBB theme with the error of Youtube Videos not loading for MyBB.