It is not possible to 'Undo' or 'Re-open' an external operation that is initiated.

When the external operations has a status higher than status 2, a purchase order (SPO) has been generated.

If the external operation has not been started by pressing Start on the external OPR, it is possible to cancel the purchase order (SPO). 

When the purhace order (SPO) is cancelled, it is possible to action the 'Undo Release' on the external operation (OPR) in the production work order (PWO).

If the purchase order (SPO) is sent, the operation started and you need this cancelled, you will have to contact JHC for assistance.