[1.16.5] How to sync slots in custom GUI?

[RInventor7]

Hi! I have created a custom TileEntity. The container has 1 slot and 1 button. I enter any item to that slot. If I press the button, it should play a sound and print out how many items are in the slot (if the slot is not empty). Doesn't matter which items/blocks I put into the slot or how many of them I put into the slot, after pressing the button the sound is NOT played and the count printed out is always 0. Why is that so? And how to fix this? Thanks. 

Edited May 27, 2022 by RInventor7

[RInventor7]

Menu https://pastebin.com/aZ0u23rw

Screen https://pastebin.com/wPEg6wRE

Events https://pastebin.com/BbAeBGNw

Block https://pastebin.com/SCA22h6i

TileEntity https://pastebin.com/nBsb7AKS

Screen functions https://pastebin.com/V92Qn8zr

[RInventor7]

ATMButtonMessage https://pastebin.com/iPtCZ4jM

I play the sound in ATMEvents lines 34 and 38

The printed output in ATMEvents line 26 shouldn't still be 0 when there are 64 items in the slot??

Edited May 27, 2022 by RInventor7

[RInventor7]

There happened to be a few simple logic mistakes. I now get the correct sound out and a correct value is printed out on server

Edit: Client it is still 0 for some reason. Because the functions checking the item count in a slot ware running only on server.

Edited May 27, 2022 by RInventor7