The error is more of a warning to say the sort is not what calibre is expecting.
If you want to use that method or sorting for all authors/books, then you need to change the defaults , see
https://www.mobileread.com/forums/sh...70&postcount=3
And if you want to control how the titles are sorted, see
https://www.mobileread.com/forums/sh...41&postcount=3