Hello,
you cannot slit a boundary of type Interface. So I suggest you convert it to type wall (in the Outline view, right click > Type > Wall). Then, undertake the TUI command:
define/boundary-conditions/modify-zones/slit-face-zone [face zone id]
After this, you should see 2 face zones with the same name (but the second will have slit in the name). Create a mesh interface, and the overlapping face zones will be modified to Interior.
You are navigating away from the AIS Discovery experience