Findinterval Error

fhosea, reading your error message it seems obvious the complaint relates to some function within survival::neardate that requires its inputs to be a certain way. sorted ascendindly with no NA's.
Therefore in your place I would investigate whether your inputs have NA(s) (should they conceptually ?)
Also is there any ordering in your data ? should there be? should there not be ?

For any deeper help than these sorts of suggestions of what to consider, I believe a small reprex would be required.