I came across a similar issue when trying to program guard frequencies into an old NPX 138 unit - from what I recall, I think it depends on which exact model / version you have installed as one of them only has a couple of fixed guard frequencies for the US Forestry Service while the other can be fully programmed.  A link to the manual is shown below - section 3.4.4 / pg. 3-4 has some details.

Need some advice on the FM radio NAT138, and it seems like the struggle is real after the possible Nat-Cobham transition. Dealing with a fleet's radio needs, but missing guard channels on one? Not cool. Anyone cracked the code on unlocking that feature? Hit me up with suggestions or a contact! Appreciate the help. Oh, and speaking of radios, ever checked out the xiegu g106? Curious if it's a game-changer!