far as i know all yale orderpickers in this series have a slack chain switch located on the chain anchor attached to the platform on the mast side. This is a safety feature incorporated into all thier orderpickers from the EAN series up to the current series.
one on each chain wired in series with each other so if any ONE chain goes slack it stops the truck and displays the slackchain error on the display... for the reason as stam stated, to keep the platform from falling and giving the operator a chance to correct it by only allowing him to raise the mast removing the slack from the chains before doing anything else.

this is a SAFETY SWITCH incorporated into these units by design and to meet OSHA safety standards for industrial truck safety and SHOULD NOT be bypassed AT ALL.
if it is not working correctly you should fix it

now if by some slim chance this unit does not have them and i would be VERY sure if they were ever on there before or not before proceeding, THEN there may be a problem in the programing of the vcm.
By default the 'slack chain' function is enabled according to the manual because as i mentioned yale made this a 'safety standard' on all their order pickers and as far as i know it is not an option anymore. The schematics for truck electrical options do not include these switches as optional , they show them as 'standard' components on the wire guided trucks and the standard trucks both.

So to make a long story short, check the wiring, the switches, the connections and adjust the switch or replace if bad or repair wiring.
And if by the slim chance the switches are not on this truck, (you have previously verified they were never on there from the factory) and you determine the vcm program has to be adjusted to disable this error then call a 'certified' Yale tech in to make the change, DO NOT attempt to do this on your own.

Good Luck