I have a reservation today for coach E. There is no coach E on the train.
Then your ticket is invalid. You must leave the train at the next station and wait for the next train that does have a Coach E.

More seriously though, is the seat reservation system working? The seat reservation system recognises when a 9-car is used instead of a 10-car, and it should move First Class reservations in Coach E to the equivalent seat numbers in Coach K. The
TM▸ should advise people of this as in the olden days on
HSTs▸ when a coach was missing and reservations were moved to the 'unreserved' carriage.