Make sure you are using the latest version of Winrar. You need Winrar v.5 to unrar RAR5 files.
If you still can't unrar after updating to latest version, then the RAR file itself might be corrupt. To confirm this, try unrar-ing other RAR files
If it is indeed corrupt, then you can try this RAR repairing software.
If none of the above help, then you'll just have to move on...