Check that the file type and size is within the specifications listed in this article.
Note: Some video file types may appear to be supported but depending on how they were created and the codec used they may not be able to imported. One way of preventing this is to convert the file into a .MP4. If you are unsure how to do this, you can download and convert it with Handbrake, a free file converter.
At times, your browser will remember old information which is not allowing you to import content. To reset this remembered information, clear your cache.
Check your proxy server, firewall and malware too see if they are blocking the file from uploading. Some times this is easier to test using a different computer that is not on the same network. Your IT department can provide additional information.
Contact email@example.com. Provide as much information as possible, including that you have tested the steps above, a screen shot of the settings and the original file.