This information is located in several places in the documentation, but takes awhile to find.
yaz_sort() only works for fields that have an associated sort register. Failure to link a sort register will generate the error "Cannot sort according to sequence".
The sort register is created in the default.idx file and referenced in the *.abs file. This is the sort register setting in default.idx:
# Sort register (no mapping at all)
In the above code the "s" register is setup as the sort register. Charmap string.chr defines the string conversion for diacritics.
This is the reference to the sort index in the *abs file:
elm (2,1) title !:w,!:p,!:s
In the above line the "!:s" causes a sort register to be created for the field.
Once the above changes have been made, reindex all the source records and restart the server. Check for a "sort<fieldid>-0.mf" file in the zebra data directory to verify operation. yaz_sort() should now work properly.