There’s something going on where you get stuck with 1 supply when trying to repair a door, the message says you need more supply but your UI says you still have 1. You do still have 1, and the way to get rid of it is to dump it on a siege item of some sort. I find it easiest to look for an oil/mortar or some sort of wall siege to drop that last one.
I haven’t bugged this because I’m not sure how to duplicate it, it happens to me when I pick up 10 or 15 so I can’t narrow it down to either of those as the culprit. The only thing I can say for sure is that it is always a wall or door that leaves you stuck at 1, yet to see it happen on siege of any sort.
Also, when you pick up 15 supply, watch your supply amount because lots of times you use 10 then the other 5 you have to hit repair again to use them. If you don’t pay attention, you are still only using 10 supply.