If I understand correctly, you have an attribute that is not available in all records. When in the Algolia dashboard, you try to add the attribute to the searchable attributes list but the attribute does not show up in the dropdown list. Is that correct?
Only attributes available in all records will show in the list. However, you can type the attribute in. It will give you a warning that it’s not in all the records, but it will still use that attribute on the records that do have that attribute available.
It’s always best to have the attribute available on all the records to get better results, even if it’s just an empty attribute.
You can also add the attribute to the searchable attributes in your code: