Add item to a chest behind a dispenser

[daknus]

Hello,

I am trying to make a script for an auto-miner for my private SkyBlock server.

What the miner does, is removes the block in front of it and drops the mined block behind it. This feature is sort of annoying, because the drop location is centered around the corner of a dispenser so I decided to change my plan a bit: The mined block will be put into a chest behind the dispenser.

Here is my script:
https://pastebin.com/mG9QsuAB

And this is the error:
https://pastebin.com/9CT76w5E

What is the cause of this issue?

 

[Donut]

add 1 of {_br} to the inventory of the block behind {_b}