Sigil's Metadata Editor does support adding calibre::series metadata but only under epub2.
Epub3 does not support them as epub3 has its own official collection and series metadata that Sigil supports.
Could you be editing an epub3 instead of an epub2? If so you should use the official epub3 series metadata.
https://www.w3.org/TR/epub-33/
Search for "belongs to collection" and "group-position"