If the extraction fails, it usually means one of the parts (like part 6) was downloaded incorrectly and needs to be re-downloaded.
The .part6.rar extension means this is the sixth piece of a larger file that was split into multiple parts to make it easier to upload or download.
If you only have part 6, you cannot open it or see the content. You must find parts 1 through 5 (and any subsequent parts) to reconstruct the file.