Can't set or get the variable

I'm trying to make my mob set target's (target is a player) variable with following: https://pastebin.com/515J7K6J

But it outputs and error: https://pastebin.com/qBVVj5Py

I also tried using trigger.ddddd and trigger.var.ddddd instead of target.ddddd and target.var.ddddd, same result. The config was made based on variable tutorial on this website. I used this in order to make a custom player leveling based on variables. Thanks for viewing, i really would like for this problem to get resolved as fast as possible.