First make sure you have enough space to burn a DVD.
You will need to make at least 4 GB of hard drive space available on drive ''C'' to burn a DVD. For a full 80-minute DVD, you should have at least 6 GB of free hard drive space. Otherwise, there will not be enough space for the temporary files required for DVD burning.
Then follow the instructions from the link below:
If the issue persists, please re-install RealPlayer as described here: https://customer.real.com/hc/en-us/articles/204040013-Uninstall-and-reinstall-RealPlayer-on-Windows