You received this error message because the media file you were trying to add to your presentation could not be uploaded.
First, please make sure that the file you are trying to add is of a format supported by Prezi.
Below are a few sample files you can test adding to your presentation.
If you have problems adding any of these sample files to your presentation, then the issue might be connected to your browser or some web security software on your end.
To resolve this, please
try changing browsers. Our website supports all major modern browsers, but for the best experience, we recommend using the latest version of Firefox, Chrome or Safari.
*Note: If you are experiencing this issue from a protected network, you may need to ask the responsible IT person to make these adjustments for you. This is common if you are trying to access Prezi.com from an educational institution or a company network.
If you can add a sample file but cannot add your file that is of the same format, then the error is most probably due to your file being corrupted or in an unsupported format.
For more information please see these articles in our Knowledge Base:
General advice on firewalls and security settings relating to Prezi
System requirements for Prezi