Can anyone shed any light on why we might get "cannot sort by field 'NLSortOption [field=commercecategoryseq, direction=desc]' errors browsing commerce categories with a new site?

Essentially what's happening is that it's trying to sort by a field that has not been set up to be used as a sort field. You need to add it. Go to setup website > search index > sort fields and add commercecategory as an option

Thanks Steve.