Restart Windows in Safe Mode (with Networking). Run the installer as Administrator. Safe mode stops third-party antivirus and background apps from interfering with unarc.dll .
FitGirl repacks are highly compressed. If you download via torrent or direct link and your connection stutters, even one bit out of place corrupts the whole archive. Reddit veterans insist on or using a download manager like IDM or JDownloader2 to verify checksums. unarc dll returned an error code: -1 reddit
At that point, stop wasting time. Download the repack (which uses a different compression algorithm less prone to -1) or grab a pre-installed "Portable" version of the game. No decompression means no error. Restart Windows in Safe Mode (with Networking)