Since 23.05 it is not possible to add MARC fields to search fields in the Mapping table in staff interface.
Administration > Search engine configuration (Elasticsearch)
Tab Bibliographic records:
Add a new line , (e.g. abstract > 942v).
See that in drop down for 'Sortable' you can select "1", "0", or "Undef" instead of "Yes" or "No".
Click the "Add" Button - the line appears (last entry)
" You attempted to delete all mappings for a required index, you must leave at least one mapping
An error occurred when updating mappings: . "
The failure to save the search mappings when adding (or deleting) a mapping appears to be resolved by the patch for bug 34625.
That just leaves the Undef/1/0 rather than Yes/No for the sortable column when adding a new mapping as the remaining issue.
I just tested and believe this was fixed by patches on other bugs. The sortable pull down now shows Yes/No.