Diagnosis
The value of the subtotal or total is locked. You try to lock the valueof one of the two remaining input ready summands. This is not possible.

System Response
When you enter a value in a data cell with a locked total or subtotal,the change in value has to be transferred to the other unlockedsummands. This means that at least one other (so, altogther at leasttwo) data cells must remain ready for input if the total is locked.
For a total with n summands, a maximum of n-2 summands can be locked atthe same time.