You do not have permission to create files in this gallery.

it wont let me upload on the demo site either, is it what im trying to upload?

The demo site did not have video uploads enabled. Once video uploads were enabled and the maximum file size parameter adjusted the below sample mp4 video uploaded fine.

www.quirksmode.org/html5/videos/big_buck_bunny.mp4

Please ensure you've enabled video uploads by navigating to CB > Plugin Management > CB Gallery > Parameters > Videos > Create and set "Upload" to "Enable".

I downloaded the mp4 you showed on this post and uploaded to my site and it worked and played well. I have other mp4 files I have uploaded and you can only see a black or grey windows and only hear sound. Why some mp4 files work and other ones dont work? I need to solve this problem. Thanks!

Sounds like your videos were encoded incorrectly. See the below for browser HTML5 supported media formats and their supported codecs.

developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats

Believe your MP4 must be encoded with H.264 codec or will not work.

CB Gallery does not do any transcoding. So it's not going to be like YouTube where you upload whatever you want and it fixes the encoding. CB Gallery will never be able to do this as transcoding is too heavy for most servers and rarely is ffmpeg installed to even do the transcoding.