The error you are having "this archive is either in unknown format or damaged" means the .rar archive is corrupted. You must repair the archive to resolve this error. Why don't you repair it with WinRAR? It is very simple and effective. Follow the steps shown below:
Open WinRAR > *click on *File tab > Open archive.
Select corrupt .rar archive and click on Open.
Now click on Tools tab > Repair archive.
Check "Treat the corrupt archive as RAR" box and click on OK.
The select .rar archive will be repaired within a few seconds. But sometimes a corrupt archive can't be repaired with this technique if corruption is severe. In such situation you can try some third-party recovery apps for corrupt .rar archives. I will suggest you to use **broken link removed** to repair your corrupt .rar archive and to recover data from it. It also supports recovery of corrupt .zip, .tar, .tar.gz and 7z files. Try its free demo version first.
Thanks.