It looks like you’re using a Query operation (which expects rows) to invoke something that definitely does not have a query. This won’t work; did you mean to use an Execute API, or fetch something?

Bad:

var id = conn.QuerySingle<int>("""
    insert SomeLookupTable (Name)
    values ('North')
    """);

Good:

var id = conn.QuerySingle<int>("""
    insert SomeLookupTable (Name)
    output inserted.Id
    values ('North')
    """);

Also fine:

conn.Execute("""
    insert SomeLookupTable (Name)
    values ('North')
    """);