The 9840 code tape can only be made from a drive that is in FULL code mode. Go to the drive configuration menu, and scroll through until you see full code? and toggle the Y for yes. Then scroll through until save/ipl comes up and hit select. After IPL, go to the drive menu and scroll to make data tape, hit select. Load your blank tape and wait for it to be set for data tape, it will eject. Then, scroll to make code tape, hit select, and load the data tape.
If this doesn't work, you need to have a drive that had code updated while it was in full code mode to create a code tape. Then, to update from a code tape, go to drive menu, and select IPL from tape. Load you code tape and the op panel should say processing. It will eject when done.