Sara's Inventory Tutorial

help i get this error

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ERROR in action number 1

of  Step Event0 for object oMouse:

Variable <unknown_object>.inventory(100004, 2) not set before reading it.

 at gml_Script_InvSwap (line 37) -        var _itemFrom = objectFrom.inventory[slotFrom];

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gml_Script_InvSwap (line 37)

gml_Script_anon@1015@gml_Object_oMouse_Create_0 (line 56) -               if (slotHover != -1) InvSwap(invDrag, slotDrag, invHover, slotHover);

gml_Object_oMouse_Step_0 (line 2) - state();

hey Shaun, i really want to know how to make the inventory system from clash royale. If you are not  familiar with the games inventory,  it has equip slots for cards,  a upgrade system for cards where if you have multible of 1 card type you can upgrade the card to its next level. Also there is a way of seeing the stats of the cards, also they have a neat shop system.  I think it will  be very useful to have a similiar inventory as clash royale in my game,  knowing the succes the game has had. Im willing  to pay you if you think it will be fun to make such a interesting inventory :) have a nice day!