what Does SIGIL_shreds not found mean?
can you add: the lost episodes of doom & back to Saturn episodes 1 to 3 please?
Sigil_shreds.wad is the extra WAD file included with the paid version of Sigil that includes the Buckethead soundtrack. WadSmoosh reporting it not being found isn't an error, it's just telling you which source wads it can and can't find.
Regarding BTS etc, from the ZDoom forum thread:
"Note: With the sole exception of SIGIL, non-official content, even stuff that was sold at retail by random companies like the Lost Episodes, is not supported! People in this thread have customized WadSmoosh to do this but it's outside the scope of what I wrote WadSmoosh to do and I have no intention of supporting it. Also please note that you don't need WadSmoosh to merge non-commercial content! Use a WAD editing tool like SLADE3 instead."