You are probably trying to burn WebM files. We have resolved this issue in our latest version. Our latest version of RealPlayer is v.18.104.22.168. To install this, please follow these steps:
1. Open RealPlayer
2. Click the RealPlayer logo on the top left corner and click "Check for updates"
3. Follow the online instructions to install the latest version
Once installed, please restart your computer and try to burn the files again. If you still have the same issue, please provide me the format of the files you are trying to burn to a DVD