Hello.
I have a simple table (item) that list the name of these items and the count of each item in inventory. User select on the screen the specific item that he wants to add in inventory from $(picklist) and he inputs the quantity of the quantity of that item to be added to the inventory table. I want to take the value from $(input field) on this screen and add it to the existing value of [item quantity] in tthe inventory table. I have the same problem for substract items from inventory table.
I welcome any and all guidance, or alternative means to achieve a similar results.
Hi @Sece-44da, welcome to the community! 
I'm happy to help 
In your "write to" field, you'll want to specify which row you are trying to update in your inventory table. To do so, you'll use the FINDROW() function to match the item in the inventory table that you want to update the quantity for.
Depending on how your tables are set up, your formula may look something like:
=FINDROW(InventoryTable, "InventoryTable[Item]=[$PicklistItem]")[Quantity]
Hope that helps! Let us know if you have any other questions. 
Hello.
Thanks for your feedback but it's not working yet. I have some syntax error ("#value!" for "take data from" and "#name?" for "write to").
I send you a new screenshot with formula update and my inventory table.
Big thanks for any help ! 
Hi @Sece-44da, thanks for the screenshot
Looking at your formula, you'll want to change structure of your syntax. The structure would be:
=FINDROW(InventoryTable, "InventoryTable[ItemColumnInTable]=[$PicklistItemInScreen]")[QuantityColumnInTable]
From what I can see, you can try something like:
=FINDROW(Inventory, "Inventory[Item]=[$Picklist]")[qty]
You'll also want to make the first column in your Inventory table a rowlink & picklist format. (The first item is blue meaning it is a rowlink; you'll want each item in that column to be a rowlink).
Let us know you have any other questions!
Hello !
It's working now, thanks a lot !
I have update "write to" like you said and I have update "take data from" --> "=FINDROW(Inventory, "Inventory[Item]=[$Picklist]")[qty]+$[inputfield]"
If I would to add a new condition in findrow (like city), the formula can be :"=FINDROW(Inventory, "Inventory[Item]=[$Picklist]","Inventory[city]=[$Picklistcity]")[qty]" ?
Regards.
Great, I'm happy that helped, @Sece-44da 
If you wish to add another condition to the formula, you'll use the AND operator. This is how the formula would look:
=FINDROW(Inventory, "Inventory[Item]=[$Picklist] AND Inventory[city]=[$Picklistcity]")[qty]
Hope that helps