Though RealPlayer can play most popular video formats, there might be some formats that are not compatible with RealPlayer. In addition, your Windows may be missing some codecs to play certain video types.
To know what the file format is, you can follow these instructions:
1. Open RealPlayer.
2. Right click on the video you are getting the error and click Locate File. (This will open the folder where the actual file is stored)
3. Right click on the actual file and click Properties.
Please look at the Type of File and let us know what it reads.
In the meanwhile, installing the K-Lite codec pack often helps to fix issues related to playback. You can install it from the link below:
After installing the codec, restart your computer and try playing the videos again.