FreeArc is excellent but no longer actively maintained. Newer tools sometimes ignore minor header corruption.
Try these alternatives:
If you have FreeArc installed:
arc t archive.arc
This tests integrity. If it fails immediately with the same message, file header is missing or wrong. FreeArc is excellent but no longer actively maintained
Before applying a fix, it helps to understand why the error occurs. The error generally happens for one of three reasons: This tests integrity