Ammunition with pluses

[LiamChgo]

I was attempting to give a character some Arrows +2 for the game I am currently running. Under the "Gear" tab, where I can find the basic arrows, there is no option for any with pluses. Under the "Weapons" tab, there are options for "Ammunition +1", "Ammunition +2", and "Ammumition +3".
I choose the +2 option, and I'm asked to pick the base item (Arrows) and to assign a custom name to it (Arrow +2). It then is added to my weapon list, but in red. When I click on the ! that appears in the upper right corner, I get the message that I need to "click on the edit button and assign a weapon to use for this item", yet when I click on the edit the only options I get are for the base item and for the name.