Board Game Arena

Hello All, can someone explain to me how does the Market Stall wqrk, please?

Its description says:
"After the field phase of each harvest, you can exchange 1 grain plus 1 fence (both from your supply) for 5 food"

I was expecting, that if I have 1 grain in my storage (not on the field) + I used 14 of 15 fences (so 1 is still available for use) then I will be able to use this card. But after field phase, during feeding phase I cannot see any feature to replace 1 grain + 1 fence => to 5 food.

How to activate it?
Or do I need to have 1 wood in my storage (to make 1 fence from it) ?
Or is it not (properly) implemented?

You have to use fence pieces available to you in your supply / tray / storage, as in ones you have not built yet. Assuming you have the grain available as well, those fence pieces are then used up, meaning they cannot be used for fencing down the line. Once you have fully fenced out and you have no pieces left in you supply you cannot use Market Stall anymore.

So it could go like this:
1) Play Market stall stage 1. Fence pieces left in supply: 15
2) Use Market stall in stage 1 harvest. Fence pieces left in supply: 14
3) Use Market stall in stage 2 harvest. Fence pieces left in supply: 13
4) Use Market stall in stage 3 harvest. Fence pieces left in supply: 12
5) Fence tight, making 4 small pastures using the available 12 fence pieces. Note that you couldn't have fenced more - no more pieces left.
6) Market stall cannot be used from this point on as you have used up all your fence pieces.